What are the main classes of antihistamines. How do antihistamines work to treat histamine-mediated conditions. What are the FDA-approved and off-label uses for H1 and H2 antihistamines. What is the mechanism of action for H1 and H2 receptor antagonists. What are the key differences between first and second-generation antihistamines.

Understanding Antihistamines: Types and Mechanisms

Antihistamines are a diverse class of pharmaceutical agents used to treat various histamine-mediated conditions. They are broadly categorized into two main classes based on the histamine receptors they target: H1 receptor antagonists and H2 receptor antagonists. Each class has distinct therapeutic applications and mechanisms of action.

H1 Receptor Antagonists

H1 antihistamines are primarily used to manage allergic conditions and symptoms. They are further subdivided into first-generation and second-generation agents, with key differences in their pharmacological properties:

• First-generation H1 antihistamines: These can readily cross the blood-brain barrier, affecting both central and peripheral histamine receptors.
• Second-generation H1 antihistamines: These agents selectively target peripheral histamine receptors, minimizing central nervous system effects.

H2 Receptor Antagonists

H2 antihistamines are primarily used to manage gastrointestinal conditions associated with excess stomach acid production. They work by blocking histamine’s action on parietal cells in the stomach.

Do H3 and H4 receptor antagonists have clinical applications? While compounds that bind to H3 and H4 receptors exist, they currently lack specific clinical benefits in human medicine.

FDA-Approved Indications for Antihistamines

Antihistamines have a wide range of FDA-approved indications, reflecting their versatility in managing various histamine-mediated conditions.

H1 Antihistamine Indications

The FDA has approved H1 antihistamines for numerous conditions, including:

• Allergic rhinitis
• Allergic conjunctivitis
• Allergic dermatological reactions
• Sinusitis
• Urticaria
• Angioedema
• Atopic dermatitis
• Bronchitis
• Motion sickness
• Nausea and vomiting

H2 Antihistamine Indications

H2 antihistamines are primarily approved for conditions related to excessive stomach acid production, such as:

• Gastroesophageal reflux disease (GERD)
• Peptic ulcer disease
• Zollinger-Ellison syndrome
• Gastritis

Are there any off-label uses for antihistamines? Yes, some common off-label uses include insomnia for H1 antihistamines and indigestion for H2 antihistamines. In cases of refractory urticaria that doesn’t respond to H1 antihistamine therapy alone, a combination of H1 and H2 antihistamines may be used.

Mechanism of Action: How Antihistamines Work

Understanding the mechanism of action of antihistamines is crucial for appreciating their therapeutic effects and potential side effects.

H1 Receptor Antagonists

H1 antihistamines work by antagonizing histamine at H1 receptors, thereby preventing histamine-induced effects such as:

• Increased vascular permeability
• Fluid movement from capillaries into surrounding tissues
• Vessel dilation and swelling

By blocking these effects, H1 antihistamines effectively reduce allergy symptoms and related manifestations.

H2 Receptor Antagonists

H2 antihistamines target the H2 receptors on parietal cells in the stomach. Their mechanism involves:

• Blocking histamine binding to H2 receptors
• Preventing the increase in cyclic adenosine monophosphate (cAMP)
• Inhibiting the activation of protein kinase A
• Reducing the phosphorylation of proteins involved in hydrogen ion transport

This cascade of events ultimately leads to a decrease in stomach acid (HCl) secretion.

How do first-generation and second-generation H1 antihistamines differ in their mechanisms? First-generation antihistamines can cross the blood-brain barrier and affect both central and peripheral H1 receptors. In contrast, second-generation antihistamines primarily target peripheral H1 receptors, resulting in fewer central nervous system effects.

Pharmacokinetics and Duration of Action

The pharmacokinetics of antihistamines play a crucial role in their clinical efficacy and dosing regimens.

First-Generation H1 Antihistamines

• Duration of action: Typically 4 to 6 hours
• Metabolism: Liver, via the P450 cytochrome system

Second-Generation H1 Antihistamines

• Duration of action: 12 to 24 hours
• Metabolism: Also primarily in the liver via the P450 cytochrome system

Why do second-generation H1 antihistamines have a longer duration of action? This extended efficacy is due to their selective binding to peripheral histamine receptors and slower metabolism, allowing for once-daily dosing in many cases.

Common Antihistamine Drugs and Their Uses

Several antihistamine drugs are commonly used in clinical practice, each with specific indications and characteristics.

H1 Antihistamines

First-generation H1 antihistamines include:

• Diphenhydramine (Benadryl)
• Chlorpheniramine
• Hydroxyzine

Second-generation H1 antihistamines include:

• Cetirizine (Zyrtec)
• Loratadine (Claritin)
• Fexofenadine (Allegra)
• Desloratadine (Clarinex)

H2 Antihistamines

Common H2 antihistamines include:

• Cimetidine
• Famotidine (most commonly used H2 antihistamine in the United States)
• Nizatidine

Why was ranitidine removed from the market? Ranitidine was withdrawn due to concerns about potential contamination with N-nitrosodimethylamine (NDMA), a probable human carcinogen.

Clinical Considerations and Adverse Effects

While antihistamines are generally safe and effective, they can cause various side effects and have important clinical considerations.

First-Generation H1 Antihistamines

Common side effects include:

• Sedation and drowsiness
• Cognitive impairment
• Dry mouth
• Blurred vision
• Urinary retention

Second-Generation H1 Antihistamines

These generally have fewer side effects, but may still cause:

• Mild sedation (less common than with first-generation agents)
• Headache
• Dry mouth

H2 Antihistamines

Side effects are generally mild but may include:

• Headache
• Dizziness
• Constipation or diarrhea

Can antihistamines interact with other medications? Yes, antihistamines can interact with various drugs, including other central nervous system depressants, anticholinergic medications, and certain antifungals and antibiotics. It’s crucial to review a patient’s complete medication list before prescribing antihistamines.

Special Populations and Contraindications

Certain populations require special consideration when using antihistamines, and there are specific contraindications to be aware of.

Pregnancy and Lactation

The safety of antihistamines during pregnancy and lactation varies:

• Some second-generation H1 antihistamines (e.g., loratadine, cetirizine) are considered relatively safe during pregnancy
• First-generation H1 antihistamines should be used with caution due to potential sedative effects on the fetus
• H2 antihistamines are generally considered safe during pregnancy, with famotidine being the preferred option

Pediatric Use

Antihistamine use in children requires careful consideration:

• Many second-generation H1 antihistamines are approved for use in children over 2 years of age
• First-generation H1 antihistamines should be used cautiously due to potential CNS effects
• Dosing should be based on weight and age

Geriatric Use

Older adults may be more susceptible to antihistamine side effects:

• First-generation H1 antihistamines can increase the risk of falls and cognitive impairment
• Second-generation H1 antihistamines are generally preferred in this population
• H2 antihistamines may require dose adjustments in patients with renal impairment

What are the main contraindications for antihistamine use? Contraindications include hypersensitivity to the specific antihistamine, severe liver disease (for some antihistamines), and certain heart conditions (for some first-generation H1 antihistamines). Caution is also advised in patients with glaucoma, prostatic hypertrophy, and certain neurological conditions.

Emerging Research and Future Directions

The field of antihistamine research continues to evolve, with new findings and potential applications emerging.

Novel Antihistamine Formulations

Researchers are exploring new formulations to enhance efficacy and reduce side effects:

• Intranasal antihistamines for more targeted allergy relief
• Combination products that incorporate antihistamines with other active ingredients
• Extended-release formulations for improved convenience and compliance

Potential New Indications

Ongoing studies are investigating the potential use of antihistamines in various conditions:

• Neurodegenerative disorders
• Certain types of cancer
• Autoimmune diseases

H3 and H4 Receptor Research

While not currently used clinically, research into H3 and H4 receptor antagonists is ongoing:

• H3 receptor antagonists are being studied for potential cognitive-enhancing effects
• H4 receptor antagonists show promise in inflammatory and allergic conditions

How might future antihistamine research impact clinical practice? Advances in antihistamine research could lead to more targeted therapies with fewer side effects, expanded indications for antihistamine use, and potentially new classes of drugs targeting H3 and H4 receptors. This could significantly enhance our ability to manage histamine-mediated conditions and potentially address other medical challenges.

In conclusion, antihistamines remain a crucial class of medications in managing a wide range of conditions. Their diverse mechanisms of action, broad indications, and generally favorable safety profiles make them invaluable tools in clinical practice. As research continues to uncover new applications and refine existing formulations, the role of antihistamines in medicine is likely to expand further, offering new therapeutic options for patients and clinicians alike.

Antihistamines – StatPearls – NCBI Bookshelf

Khashayar Farzam; Sarah Sabir; Maria C. O’Rourke.

Author Information and Affiliations

Last Update: December 27, 2022.

Continuing Education Activity

Antihistamines are a pharmaceutical class of drugs that act to treat histamine-mediated conditions. There are two main classes of histamine receptors: H-1 receptors and H-2 receptors. Antihistamine drugs that bind to H-1 receptors are generally used to treat allergies and allergic rhinitis. Drugs that bind to H-2 receptors can treat upper gastrointestinal conditions that are caused by excessive stomach acid. This activity reviews the indications, contraindications, activity, adverse events, and other vital elements of antihistamine therapy in the clinical setting as relates to the essential points needed by members of an interprofessional team managing the care of patients with conditions that respond to histamine receptor blockade.

Objectives:

• Summarize the mechanism of action of the different histamine receptor blockers.

• Identify the approved and off-label indications for the different histamine receptor blockers.

• Describe the adverse event profile and contraindications of the members of the antihistamine class.

• Outline interprofessional team strategies for improving care coordination and communication to advance appropriate clinical outcomes with antihistamine therapy to treat indicated conditions, leading to optimal patient outcomes.

Access free multiple choice questions on this topic.

Indications

Antihistamines are a pharmaceutical class of drugs that act to treat histamine-mediated conditions. There are two main classes of histamine receptors: H-1 receptors and H-2 receptors. Antihistamine drugs that bind to H-1 receptors are generally used to treat allergies and allergic rhinitis. Drugs that bind to H-2 receptors treat upper gastrointestinal conditions that are caused by excessive stomach acid.[1]

H-1 antihistamines are further classified according to first and second-generation agents. First-generation H-1 antihistamines more easily cross the blood-brain barrier into the central nervous system (CNS), whereas second-generation H-1 antihistamines do not. The first-generation drugs will bind to both central and peripheral histamine-1 receptors, whereas second-generation drugs selectively bind to peripheral histamine-1 receptors; this leads to different therapeutic and side effect profiles.[2] 

FDA-approved Indications

H-1 Antihistamines
[3]

• Allergic rhinitis

• Allergic conjunctivitis 

• Allergic dermatological reaction(s)

• Sinusitis

• Urticaria

• Angioedema

• Atopic dermatitis

• Bronchitis

• Motion sickness

• Nausea

• Vomiting

H-2 Antihistamines
[4]

Non-FDA-approved uses for H-1 antihistamines include insomnia, and for H-2, antihistamines include indigestion. Use of dual H-1 and H-2 antihistamines has been used for refractory urticaria that fails therapy with an H-1 antihistamine.  

There are two other classes of histamine receptors: H-3 and H-4. While compounds exist that bind them, there is no specific clinical benefit to clinicians using those compounds in humans.

Example Drugs

H-1 Antihistamines

H-2 Antihistamines

• Cimetidine

• Famotidine (most common H-2 antihistamine that is used in the United States)

• Raniditine (removed from market)

• Nizatidine

• Roxatidine (not available in the USA)

Mechanism of Action

Histamine (an endogenous chemical messenger) induces an increased level of vascular permeability, which leads to fluid moving from capillaries into the surrounding tissues. The overall outcome of this is increased swelling and dilation of vessels. Antihistamines stop this effect by acting as antagonists at the H-1 receptors. The clinical benefit is a reduction in allergy symptoms and any related symptoms.[5]

First-generation antihistamines easily cross the blood-brain barrier into the central nervous system and antagonize H-1 receptors, leading to a different therapeutic and adverse effect profile in contrast to second-generation antihistamines selectively bind to peripheral histamine receptors.  

The duration of the pharmacological action of first-generation antihistamines is about 4 to 6 hours. In contrast, second-generation antihistamines work for 12 to 24 hours. They are both metabolized by the liver using the P450 cytochrome system. 

Parietal cells in the gastrointestinal tract secrete hydrochloric acid. They undergo regulation by acetylcholine, gastrin, and also histamine. Histamine is released from enterochromaffin-like (ECL) cells. When histamine binds to the H-2 receptors on parietal cells, cyclic adenosine monophosphate (cAMP) increases, inducing protein kinase A. This action then leads to phosphorylation of the proteins that take part in the transport of hydrogen ions. Thus increased histamine leads to increased stomach acid, e.g., HCl secretion.[6]

The use of antihistamines specific to the H-2 receptor blocks the entire process and reduces stomach acid secretion. 

Administration

Antihistamine medications are generally administered orally in a tablet dosage form. Intravenous (IV) and intramuscular (IM) administration are also possible, reserved chiefly for in-patient usage for the treatment of specific conditions. Possible indications in the hospital setting may be for the treatment of an allergic reaction or for treating a dystonic reaction after administration of an antipsychotic medication.

Adverse Effects

Antihistamine medications carry a broad range of adverse effects depending on the specific class of drugs utilized. H-1 receptor antihistamines will generally cause clinically noticeable adverse effects that are dose-dependent. These side effects are far more commonly seen in first-generation antihistamines. Second-generation antihistamines do not easily cross the blood-brain barrier, and therefore their side effect profile is far more limited. In contrast to H-1 receptor antihistamines, H-2 receptor antihistamines do not commonly cause adverse effects except for cimetidine. 

H-1 receptor antihistamines have anticholinergic properties, which are adverse effect-inducing; this principally occurs only in the first generation category of antihistamines. As a whole, they are sedating but may cause insomnia in some users. Due to their anticholinergic properties, dry mouth is a relatively common adverse effect. Some users experience dizziness and tinnitus. At increasing doses, euphoria and decreased coordination may also occur, and delirium is a potential adverse effect at even higher dose ranges.[7] Antihistamines may also be cardiotoxic in some users as they have QTc-prolonging effects.[8] 

H-2 receptor antihistamines are generally well tolerated by users but do carry the risk of uncommon side effects. Gastrointestinal changes can be seen, including both diarrhea and constipation. Reports exist of fatigue, dizziness, and confusion. One specific drug in this category that may cause a range of adverse effects is cimetidine. Its antiandrogenic effects correlate with the possible occurrence of gynecomastia in men. In women, it can cause galactorrhea. Other H-2 receptor antihistamines do not exhibit the same properties as cimetidine.[9] Ranitidine was previously removed from the market in the United States due to concerns of potential contamination with a carcinogen.  

H-2 receptor antihistamines can cause inhibition of the cytochrome system, especially cimetidine, thereby leading to drug toxicity and interactions with other medications.

Patients who present with hemodynamic alterations, increased intraocular pressure or increased urinary retention should use antihistamines with caution as these conditions can become exacerbated.

Contraindications

Given the potential cardiotoxic effects of certain antihistamines, they are relatively contraindicated in any patient with QTc prolongation. Patients using other QTc-prolonging drugs require careful monitoring for further prolongation of the QTc interval due to the risk of potentially fatal cardiac arrhythmias.[8] 

Usage in pregnant women is a relative contraindication. Additionally, women who are lactating should also avoid usage of antihistamines. 

Patients with impaired renal or hepatic function should use antihistamines with caution. 

Hypertension, cardiovascular disease, urinary retention, increased ocular pressure are relative contraindications to the use of antihistamines.  

Monitoring

Doses of the antihistamines may require monitoring especially during prolonged use. Patients should be monitored for anticholinergic effects. This is especially true in elderly who are at increased risk of falls.

The cardiotoxic effects of antihistamines may be monitored on an electrocardiogram (ECG) to assess prolongation of the QTc interval.

Toxicity

There is no specific antidote used for the treatment of antihistamine overdose. However, physostigmine may be an option if a patient is experiencing delirium or other toxicity side effects due to the anticholinergic effects of the antihistamine. 

Enhancing Healthcare Team Outcomes

Antihistamines are a class of medications that can be subdivided into H-1 and H-2 categories. H-1 antihistamines, which can be further divided into first and second generations, are primarily used to treat allergic symptoms and illnesses mediated through similar mechanisms. H-2 antihistamines can lower excessive stomach acid and thereby treat acid reflux, gastritis, and gastrointestinal ulcers.  

Pharmacists serve as dispensers and educators of these medications, given the availability of antihistamines at pharmacies for off-the-shelf purchase. They have a crucial role in advising the patient to utilize the correct dose and be cautious of any contraindications and adverse effects. Nurses should be prepared to answer questions regarding these medications, as well as offer counsel on adverse effects and note their therapeutic effectiveness, and report any findings to the clinician.

Providers that recommend antihistamines, such as nurse practitioners, physician assistants, and physicians, should use caution and make sure the healthcare team is aware of the recommendation and the pharmacist can review the patient’s medication profile to determine if there are any clinically significant drug interactions, especially in elderly patients. [Level 5]

Review Questions

• Access free multiple choice questions on this topic.

• Comment on this article.

References

1.

Monczor F, Fernandez N. Current Knowledge and Perspectives on Histamine h2 and h3 Receptor Pharmacology: Functional Selectivity, Receptor Crosstalk, and Repositioning of Classic Histaminergic Ligands. Mol Pharmacol. 2016 Nov;90(5):640-648. [PubMed: 27625037]

2.

Schaefer TS, Zito PM. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): Mar 7, 2023. Antiemetic Histamine h2 Receptor Blockers. [PubMed: 30422595]

3.

Curto-Barredo L, Giménez-Arnau AM. Treatment of chronic spontaneous urticaria with an inadequate response to h2-antihistamine. G Ital Dermatol Venereol. 2019 Aug;154(4):444-456. [PubMed: 30717573]

4.

Kuna L, Jakab J, Smolic R, Raguz-Lucic N, Vcev A, Smolic M. Peptic Ulcer Disease: A Brief Review of Conventional Therapy and Herbal Treatment Options. J Clin Med. 2019 Feb 03;8(2) [PMC free article: PMC6406303] [PubMed: 30717467]

5.

Pirahanchi Y, Sharma S. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): Jul 25, 2022. Physiology, Bradykinin. [PubMed: 30725872]

6.

Heda R, Toro F, Tombazzi CR. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): May 8, 2022. Physiology, Pepsin. [PubMed: 30725690]

7.

Boley SP, Olives TD, Bangh SA, Fahrner S, Cole JB. Physostigmine is superior to non-antidote therapy in the management of antimuscarinic delirium: a prospective study from a regional poison center. Clin Toxicol (Phila). 2019 Jan;57(1):50-55. [PubMed: 29956570]

8.

Farzam K, Tivakaran VS. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): Feb 12, 2023. QT Prolonging Drugs. [PubMed: 30521285]

9.

Bowman JD, Kim H, Bustamante JJ. Drug-induced gynecomastia. Pharmacotherapy. 2012 Dec;32(12):1123-40. [PubMed: 23165798]

Disclosure: Khashayar Farzam declares no relevant financial relationships with ineligible companies.

Disclosure: Sarah Sabir declares no relevant financial relationships with ineligible companies.

Disclosure: Maria O’Rourke declares no relevant financial relationships with ineligible companies.

Antihistamine Oral: Uses, Side Effects, Interactions, Pictures, Warnings & Dosing

Uses

This combination medication is used to temporarily relieve symptoms caused by the common cold, flu, allergies, or other breathing illnesses (such as sinusitis, bronchitis). Antihistamines help relieve watery eyes, itchy eyes/nose/throat, runny nose, and sneezing. Decongestants help to relieve stuffy nose and ear congestion symptoms.If you are self-treating with this medication, carefully read the package instructions to be sure it is right for you before you start using this product. Some products have similar brand names but different active ingredients with different uses. Taking the wrong product could harm you. Ask your pharmacist if you have any questions about your product or its use.Cough-and-cold products have not been shown to be safe or effective in children younger than 6 years. Do not use this product to treat cold symptoms in children younger than 6 years unless specifically directed by the doctor. Some products (including some long-acting tablets/capsules) are not recommended for use in children younger than 12 years. Ask your doctor or pharmacist for more details about using your product safely. Do not use this product to make a child sleepy.These products do not cure or shorten the length of the common cold and may cause serious side effects. To decrease the risk for serious side effects, carefully follow all dosage directions. Do not give other cough-and-cold medication that might contain the same or similar ingredients (see also Drug Interactions section). Ask the doctor or pharmacist about other ways to relieve cough and cold symptoms (such as drinking enough fluids, using a humidifier or saline nose drops/spray).

How to use Antihistamine oral

Take this medication by mouth as directed by your doctor. If you are self-treating, follow all directions on the product package. If you have any questions, ask your doctor or pharmacist.

This medication may be taken with food if stomach upset occurs. Drink plenty of fluids unless otherwise directed by your doctor.

If you are using the liquid form of this medication, carefully measure the dose using a special measuring device/spoon. Do not use a household spoon because you may not get the correct dose. If your liquid form is a suspension, shake the bottle well before each dose.

Do not crush or chew extended-release tablets or capsules. Doing so can release all of the drug at once, increasing the risk of side effects. Also, do not split extended-release tablets unless they have a score line and your doctor or pharmacist tells you to do so. Swallow the whole or split tablet without crushing or chewing.

If you are using chewable tablets, chew each tablet thoroughly before swallowing.

If you are using a product made to dissolve in the mouth (tablets/strips), dry your hands before handling the medication. Place each dose on the tongue and allow to dissolve completely, then swallow it with saliva or with water.

Dosage is based on the product you are taking and your age, medical condition, and response to treatment. Do not increase your dose or take this medication more often than directed without your doctor’s approval. Improper use (abuse) of this medication may result in serious harm (such as hallucinations, seizure, death).

If your doctor directs you to take this medication daily, take it regularly to get the most benefit from it. To help you remember, take it at the same time(s) each day.

Tell your doctor if your condition lasts for more than 1 week, if it gets worse, or if it occurs with a headache that doesn’t go away, fever, or rash. These may be symptoms of a serious medical problem and should be checked by a doctor.

Side Effects

Drowsiness, dizziness, dry mouth/nose/throat, headache, upset stomach, constipation, or trouble sleeping may occur. If any of these effects last or get worse, tell your doctor or pharmacist promptly.

If your doctor has directed you to use this product, remember that your doctor has judged that the benefit to you is greater than the risk of side effects. Many people using this product do not have serious side effects.

Tell your doctor right away if you have any serious side effects, including: mental/mood changes (such as confusion, hallucinations), ringing in the ears, difficulty urinating, vision changes (such as blurred/double vision).

Get medical help right away if you have any very serious side effects, including: fast/irregular heartbeat, seizure.

A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing.

This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist.

In the US – Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch.

In Canada – Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

Precautions

Before taking this product, tell your doctor or pharmacist if you are allergic to any of its ingredients; or if you have any other allergies. This product may contain inactive ingredients, which can cause allergic reactions or other problems. Talk to your pharmacist for more details.

Before using this medication, tell your doctor or pharmacist your medical history, especially of: breathing problems (such as asthma, emphysema), diabetes, glaucoma, heart problems, high blood pressure, kidney problems, liver disease, seizures, stomach/intestinal problems (such as ulcers, blockage), overactive thyroid (hyperthyroidism), difficulty urinating (such as due to enlarged prostate).

This drug may make you dizzy or drowsy. Alcohol or marijuana (cannabis) can make you more dizzy or drowsy. Do not drive, use machinery, or do anything that needs alertness until you can do it safely. Limit alcoholic beverages. Talk to your doctor if you are using marijuana (cannabis).

Liquid products, chewable tablets, or dissolving tablets/strips may contain sugar or aspartame. Liquid products may also contain alcohol. Caution is advised if you have diabetes, alcohol dependence, liver disease, phenylketonuria (PKU), or any other condition that requires you to limit/avoid these substances in your diet. Ask your doctor or pharmacist about using this product safely.

Before having surgery, tell your doctor or dentist about all the products you use (including prescription drugs, nonprescription drugs, and herbal products).

Children may be more sensitive to the side effects of this product, especially excitation and agitation.

Older adults may be more sensitive to the side effects of this product, especially dizziness, drowsiness, confusion, constipation, fast/irregular heartbeat, trouble sleeping, or urination problems. Dizziness, drowsiness, trouble sleeping, and confusion can increase the risk of falling.

During pregnancy, this medication should be used only when clearly needed. Discuss the risks and benefits with your doctor.

This medication may pass into breast milk and the effect on a nursing infant is unknown. Consult your doctor before breast-feeding.

Interactions

Drug interactions may change how your medications work or increase your risk for serious side effects. This document does not contain all possible drug interactions. Keep a list of all the products you use (including prescription/nonprescription drugs and herbal products) and share it with your doctor and pharmacist. Do not start, stop, or change the dosage of any medicines without your doctor’s approval.

Some products that may interact with this drug are: antihistamines applied to the skin (such as diphenhydramine cream, ointment, spray), blood pressure medications (especially guanethidine, methyldopa, beta blockers such as atenolol, or calcium channel blockers such as nifedipine).

Taking MAO inhibitors with this medication may cause a serious (possibly fatal) drug interaction. Avoid taking MAO inhibitors (isocarboxazid, linezolid, metaxalone, methylene blue, moclobemide, phenelzine, procarbazine, rasagiline, safinamide, selegiline, tranylcypromine) during treatment with this medication. Most MAO inhibitors should also not be taken for two weeks before treatment with this medication. Ask your doctor when to start or stop taking this medication.

Tell your doctor or pharmacist if you are taking other products that cause drowsiness such as opioid pain or cough relievers (such as codeine, hydrocodone), alcohol, marijuana (cannabis), drugs for sleep or anxiety (such as alprazolam, lorazepam, zolpidem), muscle relaxants (such as carisoprodol, cyclobenzaprine), or other antihistamines (such as cetirizine, diphenhydramine).

Check the labels on all your medicines (such as allergy or cough-and-cold products, diet aids) because they may contain ingredients that could affect your blood pressure or cause drowsiness. Ask your pharmacist about using those products safely.

This medication may interfere with certain medical/lab tests (such as brain scan for Parkinson’s disease, urine drug screening tests), possibly causing false test results. Make sure lab personnel and all your doctors know you use this drug.

Does Antihistamine oral interact with other drugs you are taking?

Enter your medication into the WebMD interaction checker

Overdose

If someone has overdosed and has serious symptoms such as passing out or trouble breathing, call 911. Otherwise, call a poison control center right away. US residents can call their local poison control center at 1-800-222-1222. Canada residents can call a provincial poison control center. Symptoms of overdose may include: irregular heartbeat, hallucinations, fainting, seizures.

If your doctor has prescribed this medication, do not share it with others.

Keep all medical and lab appointments.

Do not take this product for several days before allergy testing because test results can be affected.

If you are taking this product on a regular schedule and miss a dose, take it as soon as you remember. If it is near the time of the next dose, skip the missed dose. Take your next dose at the regular time. Do not double the dose to catch up.

Store at room temperature away from light and moisture. Do not store in the bathroom. Do not freeze liquid forms of this medication. Keep all medications away from children and pets.

Do not flush medications down the toilet or pour them into a drain unless instructed to do so. Properly discard this product when it is expired or no longer needed. Consult your pharmacist or local waste disposal company.

Images

Next

Save up to 80% on your prescriptions.

Available coupons

Save up to 80% on your prescription with WebMDRx

Drug Survey

Have you ever purchased Antihistamine oral?

Yes, In the past 3 months

Yes, In the past 6 months

Yes, In the past year

Haven’t purchased but considering

Don’t plan to purchase

This survey is being conducted by the WebMD marketing sciences department.

Selected from data included with permission and copyrighted by First Databank, Inc. This copyrighted material has been downloaded from a licensed data provider and is not for distribution, except as may be authorized by the applicable terms of use.

CONDITIONS OF USE: The information in this database is intended to supplement, not substitute for, the expertise and judgment of healthcare professionals. The information is not intended to cover all possible uses, directions, precautions, drug interactions or adverse effects, nor should it be construed to indicate that use of a particular drug is safe, appropriate or effective for you or anyone else. A healthcare professional should be consulted before taking any drug, changing any diet or commencing or discontinuing any course of treatment.

how and when to use them

Contents

• 1 Which antihistamines should be used at the right time?
  • 1.1 Allergy and its symptoms
  • 1.2 How antihistamines work
  • 1. 3 Types of antihistamines
    • 1.3.1 Classical antihistamines
    • 1.3.2 Modern antihistamines
  • 1.4 First generation antihistamines
  • 1.5 Second generation antihistamines
  • 1.6 Third generation antihistamines: efficacy and safety
    • 1.6.1 What are third generation antihistamines?
    • 1.6.2 Benefits of third generation antihistamines
    • 1.6.3 What are the third generation antihistamines?
    • 1.6.4 How to take third generation antihistamines?
  • 1.7 Modern antihistamines
  • 1.8 How to choose an antihistamine
  • 1.9 Contraindications for the use of antihistamines
    • 1.9.1 Learn more about when not to take antihistamines
    • 1.9.2 Occupational restrictions
    • 1.9.3 Personality organism
  • 1.10 Rules for use antihistamines
  • 1.11 Allergy prevention
  • 1.12 Related videos:
  • 1.13 Q&A:
    • • 1.13.0. 1 What are antihistamines?
      • 1.13.0.2 When should antihistamines be used?
      • 1.13.0.3 At what age can antihistamines be used?
      • 1.13.0.4 Can antihistamines be used during pregnancy and lactation?
      • 1.13.0.5 What are the side effects of using antihistamines?
      • 1.13.0.6 Is it possible to get stuck on antihistamines?

Find out when and which antihistamines to use to treat allergic reactions and symptoms such as itching, runny nose, and red skin. Get useful tips and advice on choosing effective and safe medicines.

Allergies have become one of the most common problems in modern society. More and more people are experiencing unpleasant symptoms such as a runny nose, cough, itching, etc. In such cases, antihistamines come to the rescue – highly effective drugs that help reduce allergy symptoms and improve quality of life.

However, these drugs should not be abused or taken without the supervision of a qualified physician. Uncontrolled use of medicines can cause significant harm to health. In this article, we consider the main guidelines for the use of antihistamines – what symptoms should be eliminated by these drugs, when self-medication can be used, and when it is necessary to seek medical help.

In addition, we will look at the different types of antihistamines and their features. You will also learn about possible side effects and how to prevent them. We hope that this article will help you use antihistamines more competently and wisely and provide an effective fight against allergic manifestations.

Allergy and its symptoms

Allergy is a violation of the human immune system, in which the body begins to react incorrectly to certain substances – allergens. The most common allergens are plant pollen, pet pollen, indoor dust and fungal spores, foods, and drugs.

Some symptoms may come on very suddenly and quickly, such as in anaphylactic shock. But in most cases, symptoms appear gradually and may not occur immediately after contact with the allergen, but may occur after several hours or even days.

An allergist or an immunologist should be consulted for the diagnosis of allergy and identification of the allergen. In modern medicine, there are several diagnostic methods, including allergological tests, allergen tests, clinical studies, etc.

How antihistamines work

Antihistamines are a group of medicines that are used to combat allergic reactions. They act on histamine receptors, which are responsible for the development of allergic manifestations. Antihistamines block these receptors to prevent the release of histamine and reduce allergy symptoms.

Antihistamines may be the first line of defense in the treatment of allergic reactions. They can be used to treat symptoms such as runny nose, itching, itchy skin, redness, and watery eyes.

There are several generations of antihistamines with varying degrees of histamine receptor blocking. Some drugs strongly block receptors and can cause drowsiness, dizziness and slow reaction times, so they are not recommended to be taken before driving a vehicle or performing other activities that require maximum concentration.

Antihistamines act quickly and may provide temporary relief, but they do not have an anti-allergic effect and do not address the cause of the allergic reaction.

Types of antihistamines

There are two main generations of antihistamines: classical (first generation) and modern (second generation).

Classic antihistamines

• Diphenhydramine is a drug that relieves itching, reduces redness and swelling. It is taken orally or intramuscularly.
• Suprastin is a well-known drug that has an antispasmodic effect. It is taken orally and, according to an individual scheme, can be prescribed by a doctor for weeks or months.
• Tavegil is a drug that blocks histamine receptors and reduces allergy symptoms. It is taken orally or may be given by injection (intramuscular or intravenous) in severe cases.

Modern antihistamines

1. Loratadine – a drug that has a long-term effect, acts softer than classic antihistamines. It is taken orally.
2. Cetirizine is an effective drug with similar characteristics to Loratadine. It is taken orally, has a long effect.
3. Fexofenadine is a second generation antihistamine that has anti-inflammatory properties, does not cause drowsiness, and is fast acting. It is taken orally.

When prescribing antihistamines, an individual approach is very important and it is necessary to consult a doctor so that he selects the drug and dosage depending on the characteristics of the patient and the degree of allergic reaction.

First generation antihistamines

The first antihistamines were developed in the early 20th century to treat allergic reactions. They blocked the action of histamine, which is the main mediator of allergic reactions.

Other drugs of the first generation include suprastin, tavegil, fenkarol. They also blocked the action of histamine, but had similar disadvantages.

Although the first generation of antihistamines are still in use today, they have been replaced by second and third generations, which have fewer side effects and a longer duration of action.

Second generation antihistamines

Second generation antihistamines are selective drugs. They are recommended for the treatment of seasonal and persistent allergic rhinitis, conjunctivitis and atopic dermatitis. Unlike the first generation of drugs, they have a long-term effect and do not cause drowsiness.

Second-generation antihistamines include the following: cetirizine, fexofenadine, desloratadine, levocetirizine, rupatadine, and others. They can be used by both adults and children.

To maximize the effectiveness of second-generation antihistamines, allergen avoidance is essential. However, even if this condition is observed, a transition from a mild form of the disease to a moderate one is possible. In this case, it is possible to prescribe a combination therapy with the use of glucocorticosteroid drugs and second-generation antihistamines.

• Cetirizine is the active ingredient in Zirtek, Setrizin, Alergodil. Effective in allergic reactions of various etiologies and gives a long-term effect.
• Fexofenadine is the main component of Telfast and Allerfex preparations. It has a number of advantages over representatives of the first generation of drugs – it does not cause drowsiness and hyperactivity.
• Desloratadine – characterized by low toxicity, rapid onset of action and long-term effect, up to 24 hours, even with a single application per day.

Different drugs may be suitable for different individuals, so the choice must be determined for each individual. To do this, you need to see a doctor.

Third generation antihistamines: efficacy and safety

What are third generation antihistamines?

Third generation antihistamines are a new generation of drugs used to treat allergies. They are highly effective and have no sedative effect. The main advantage of the new drugs is the elimination of allergy symptoms without the need to increase the dosage or take additional drugs.

Benefits of using third generation antihistamines

The unique characteristic of third generation antihistamines is that they not only effectively control allergy symptoms, but also prevent the development of new allergic reactions. In addition, they do not cause drowsiness and do not adversely affect the functioning of the central nervous system.

What are the third generation antihistamines?

Third-generation antihistamines include the following: Cetirizine, Fexofenadine, Levocetirizine, Desloratadine, Ebastin, and others. Their effectiveness has already been proven and they are often prescribed by doctors for the treatment of various types of allergies.

How do I take third generation antihistamines?

Third generation antihistamine dosages may vary depending on the patient and their body characteristics. Before you start taking medications, you should consult your doctor and follow the recommendations for dosage and use of drugs. It is also important to remember that third-generation antihistamines should only be taken as directed by a doctor.

Modern antihistamines

Antihistamines are medicines used to relieve allergic reactions. There are many different drugs on the market today, differing in mode of action, speed and duration of action, as well as the degree of safety and side effects.

Examples of non-metabolite antihistamines:

• Ketotifen;
• Fenistil;
• Suprastin;
• Dimedrol.

Examples of metabolite antihistamines:

• Levocetirosine;
• Cetirizine;
• Fexofenadine;
• Loratadine.

Examples of biological antihistamines:

• Omalizumab;
• Mepolizumab;
• Benralizumab.

Different antihistamines are used depending on the type of allergic reaction and its symptoms. Non-metabolite preparations are suitable for the treatment of acute allergies of choice, metabolite preparations for long-term use, biological preparations for severe cases of allergy.

How to choose an antihistamine

Allergies are a common disease that contributes to the development of a number of symptoms, such as itching, hives, runny nose and sore throat. However, the cause of an allergy may be different in each case, so it is important to choose the right antihistamine to treat and reduce symptoms.

Before choosing a drug, you need to pay attention to the type of allergy. For example, for the treatment of seasonal allergies caused by red and yellow pimples, first-generation antihistamines such as diphenhydramine or suprastin are suitable.

If you are allergic to pollen, then you should choose a second generation antihistamine like ketotifen or cetirizine. Not only do they reduce symptoms, but they also have additional properties, such as reducing migraines and itching.

On the other hand, for the treatment of allergies, skin rashes or itching, third-generation antihistamines may be required. These drugs are more effective and have fewer side effects than first and second generation drugs.

If you are suffering from a serious condition such as anaphylactic shock, you should contact your doctor immediately, as epinephrine may be necessary in this case.

Regardless of the choice of antihistamine, follow the instructions for use and consult a doctor if necessary.

Antihistamine Contraindications

Learn more about when not to take antihistamines

Despite the fact that antihistamines are effective in combating allergic reactions, they have a number of contraindications. For example, during pregnancy and lactation, it is not recommended to take such medicines, since their effect on the child is unknown.

Also, people suffering from liver or kidney disease should take antihistamines with caution, as these organs play an important role in drug metabolism. In this case, it is necessary to consult a doctor.

Occupational restrictions

There are certain occupations that may not be compatible with the use of antihistamines. For example, drivers, aviation workers, and many others should be careful when using these medications, as they can reduce alertness and alertness. In such cases, the decision to take antihistamines should only be taken after consultation with a specialist and after an analysis of the possible risks and benefits.

Personality of the body

Some people may have an individual intolerance to antihistamines, which manifests itself in the form of side effects such as drowsiness, dizziness, impaired concentration, etc. In this case, you should consult a doctor to select the most appropriate medication or change the dosage.

Rules for the use of antihistamines

Antihistamines are one of the main tools for combating allergic reactions. They improve the patient’s quality of life by reducing allergy symptoms. However, in order to achieve the maximum effect of treatment, it is necessary to observe the correct dosage and use of drugs.

Dosage and timing of antihistamines should be determined individually. Some medications should be taken 1 to 2 hours before the onset of symptoms, others should be taken during the day or at night. Do not exceed the recommended dose, as this may lead to side effects or deterioration of health.

• Antihistamines can be taken in tablets, injections, drops, and other forms.
• Antihistamines should not be taken with alcohol as this may increase unwanted side effects.
• In the treatment of children and pregnant women, it is necessary to carefully select drugs and follow the recommended dosage.

Compliance with the rules for the use of antihistamines helps to achieve the maximum effect of treatment and minimize possible health risks. In any case, before starting treatment, you should consult a doctor and get recommendations on the use of antihistamines.

Allergy prevention

Allergic reactions can occur in people for no apparent reason, so it is very important to prevent this disease. Allergy prevention includes the following activities:

• Avoid contact with allergens. If you know what substances you are allergic to, try to avoid contact with them, especially if it is dust, pollen or animal hair.
• Clean daily. Your home needs to be clean and dust-free, so clean regularly with an allergen-fighting product.
• Maintain a healthy lifestyle. Exercise, proper nutrition and regular rest will strengthen your body and make it less susceptible to allergies.
• Take immunomodulators. Many immunomodulators help increase the body’s defenses, which can reduce sensitivity to allergens.
• Contact a specialist. If you suspect that you or your child may have an allergy, be sure to see an allergist for an examination and treatment.

Remember that allergy prevention will help you avoid many health problems, so follow these simple rules and be healthy!

Related videos:

Q&A:

What are antihistamines?

There are several generations of antihistamines. The first generation includes diphenhydramine, diphenhydramine, suprastin, etc. They quickly relieve allergy symptoms, but cause drowsiness and other side effects. The second generation of antihistamines – cetirizine, levocetirizine, fexofenadine – are less dormant and more effective in combating allergy symptoms. The third generation of antihistamines – loratadine, desloratadine, rupatadine, bilastine – have the highest efficacy and duration of action, and also almost do not cause side effects.

When should antihistamines be used?

Antihistamines should be used for allergy symptoms such as itching, redness, runny nose, hypersensitivity and other manifestations. In addition, antihistamines can help with hives, eczema, and other allergic skin reactions. Also, drugs can be used to prevent allergic reactions before medical procedures or medications.

At what age can antihistamines be used?

Most antihistamines are not recommended for children under 2 years of age. However, some drugs may be used to treat children older than 6 months. Before using antihistamines, children should consult a pediatrician and follow the instructions for use exactly.

Can antihistamines be used during pregnancy and lactation?

Antihistamines should not be used during pregnancy or breastfeeding without medical advice. Some drugs may be safe for pregnant and breastfeeding mothers, but others may harm the fetus or baby. Therefore, it is necessary to consult a specialist before using antihistamines during pregnancy and lactation.

What are the side effects of using antihistamines?

Side effects from the use of antihistamines may include drowsiness, dizziness, dry mouth, decreased concentration, etc. However, these side effects usually decrease with time or when the dose of the drug is reduced. In some cases, there may be more serious side effects such as heart rhythm disturbances, insomnia, anxiety and others, so you should follow the instructions for use and consult your doctor if any adverse reactions appear.

Is it possible to get stuck on antihistamines?

Antihistamines are not addictive. However, if the drugs are used for a long time, there may be some tolerance, when the dose must be increased to achieve the same effect. Therefore, you must follow the instructions for use and do not exceed the recommended dose.

ABC-medicine

One of the most highly specific methods for diagnosing allergic diseases is the prick test. This painless, cost-effective procedure helps to identify not only a clinically significant allergen, but also determine the degree of sensitization of the body.

What is a scratch test?

Scarification test (puncture) is a diagnostic technique used to determine sensitization (increased sensitivity of the body) to the effects of household, epidermal, pollen and fungal allergens. This test allows you to identify the patient’s allergy to various foods, pet dander, mold, soil, pollen, dust mites and others (a total of about 40 allergens). Scarification skin tests are quite simple and easily tolerated even in early childhood. Most often they are used to diagnose immediate hypersensitivity.

Prick tests

Immediately before the procedure, the area where the skin test will be performed is treated with 70% ethyl alcohol (for children in the back area, for adults – on the back of the forearm). Then marks are made on the skin at a distance of 4–5 cm from each other.

Skin testing involves the use of special disposable lancets with fixed allergens. Surface scratches 0.5 cm long are made with these easy-to-handle pointed devices.

At the same time, tests with histamine and saline are carried out in the same way (they act as positive and negative controls). In the absence of a reaction to histamine, the test will be negative even if there is an allergic pathology.

Test results are recorded after 15–20 minutes. In the presence of an allergy to a particular allergen under study, a swelling occurs at the site of the notch, similar to a mosquito bite, often accompanied by redness and itching. In some cases, a blister filled with a liquid substance forms at the site of contact with the allergen. After measurement in two directions of limited skin edema (papules), scratches are carefully wiped with alcohol.

Evaluation of the results of scratch tests

1. Negative result (-): the size of the redness at the site of contact with the allergen does not exceed 1 mm.
2. Indeterminate result (±): A papule (swelling) forms on the skin.
3. Weakly positive result (+): papule and blister up to 3 mm in diameter.
4. Positive result (++): papule and wheal up to 5 mm in diameter.
5. Strongly positive (+++): papule and blister up to 10 mm in diameter.
6. Very strongly positive (++++): swelling diameter and blister greater than 10 mm, extensive redness develops, general reactions are observed.

In some cases, prick tests give false positive or false negative results.

Causes of erroneous results

• Violation of skin allergy testing technique.
• Violation of methods of storage and transportation of allergen extracts.
• Notches too close together (less than 2–3 cm apart).
• Taking antihistamines 72 hours before the test.
• Reduced skin reactivity.

Non-specific factors affecting skin test results

• Allergen injection site.
• Local circulation. During the cold period, patients may experience vascular disorders, so it is first necessary to restore the skin temperature.
• Prolonged natural contact with the allergen, which can lead to an increase in the reaction.
• Age. In very young children and in some patients older than 50 years, the severity of skin reactions is significantly reduced.
• Daily biorhythms. The minimum deviations are observed at 11 o’clock, the maximum – at night, at 23 o’clock. Fluctuations can be 5–70%. According to experts, this condition is associated with the synthesis of endogenous hydrocorticosteroids.

Skin scarification tests are carried out in a specialized office by an allergist-immunologist who has all the necessary practical skills. The appointment of this diagnostic study is carried out taking into account indications and contraindications.

Indications

• Hay fever.
• Atopic dermatitis.
• Food allergy.
• Drug allergy.
• Eczema.
• Respiratory allergies.

Contraindications for scarification tests

• Exacerbation of an allergic disease.
• Acute infectious pathologies.
• Exacerbation of foci of chronic infection.
• Autoimmune diseases.
• Oncopathology.
• Psychosomatic disorders.
• AIDS.
• History of an acute reaction to a specific antigen.
• Pregnancy and lactation.
• Early childhood (relative contraindication).
• Treatment with antihistamines, glucocorticosteroids and mast cell membrane stabilizers.