What are the adverse effects of antihistamines?. Discover the various side effects, contraindications, and important considerations for using antihistamine medications.

Understanding Antihistamines and Their Mechanisms

Antihistamines are a class of pharmaceutical drugs that target histamine receptors in the body. Histamine is a chemical messenger involved in various physiological processes, and antihistamines work by blocking or antagonizing its effects. There are two main types of histamine receptors: H-1 and H-2.

H-1 antihistamines are primarily used to treat conditions like allergic rhinitis, allergic conjunctivitis, and other allergic reactions. They work by binding to H-1 receptors, preventing histamine from causing increased vascular permeability and subsequent swelling and dilation of blood vessels.

H-2 antihistamines, on the other hand, are used to manage upper gastrointestinal conditions caused by excessive stomach acid, such as heartburn and peptic ulcers. They work by blocking histamine’s action on the parietal cells in the stomach, reducing the production of hydrochloric acid.

First-Generation vs. Second-Generation Antihistamines

Antihistamines are further classified into first-generation and second-generation agents based on their ability to cross the blood-brain barrier. First-generation antihistamines can easily cross the blood-brain barrier and interact with central nervous system (CNS) histamine receptors, leading to a different therapeutic and adverse effect profile compared to second-generation antihistamines.

First-generation antihistamines have a shorter duration of action, typically around 4-6 hours, while second-generation antihistamines have a longer duration of 12-24 hours. Both types are metabolized by the liver using the P450 cytochrome system.

Approved and Off-Label Indications for Antihistamines

The approved indications for H-1 antihistamines include:

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

H-2 antihistamines are approved for the treatment of conditions caused by excessive stomach acid, such as heartburn and peptic ulcers.

Some off-label uses of antihistamines include the treatment of insomnia (H-1 antihistamines) and indigestion (H-2 antihistamines). Additionally, the use of dual H-1 and H-2 antihistamines has been explored for refractory urticaria that doesn’t respond to H-1 antihistamine therapy alone.

Adverse Effects of Antihistamines

Antihistamines can have a range of adverse effects, which vary depending on the specific agent and its characteristics. Some common adverse effects include:

Central Nervous System (CNS) Effects

First-generation antihistamines are more likely to cause CNS-related adverse effects, such as drowsiness, sedation, impaired cognitive function, and dizziness. These effects are less common with second-generation antihistamines, which are designed to have a lower penetration of the blood-brain barrier.

Anticholinergic Effects

Antihistamines can also have anticholinergic effects, leading to dry mouth, constipation, urinary retention, and blurred vision. These effects are more pronounced with first-generation antihistamines.

Cardiovascular Effects

Some antihistamines, particularly the first-generation agents, have been associated with QT interval prolongation, which can increase the risk of arrhythmias. This is an important consideration, especially in patients with underlying cardiovascular conditions.

Other Adverse Effects

Other potential adverse effects of antihistamines include headache, fatigue, gastrointestinal disturbances, and hypersensitivity reactions. The risk and severity of these effects can vary depending on the specific antihistamine and the individual patient’s characteristics.

Contraindications and Precautions

Antihistamines should be used with caution in certain populations, such as those with:

• Narrow-angle glaucoma
• Benign prostatic hyperplasia
• Urinary retention
• Cardiovascular disease
• Seizure disorders
• Pregnancy and breastfeeding

Healthcare providers should carefully consider the potential risks and benefits of antihistamine therapy, especially in patients with these conditions or when using antihistamines in combination with other medications that may have interactions.

Optimizing Antihistamine Therapy

To ensure the safe and effective use of antihistamines, an interprofessional team approach is essential. Healthcare providers, including physicians, pharmacists, and nurses, should work together to:

• Carefully assess the patient’s medical history and current condition to determine the appropriate antihistamine and dosage
• Monitor for adverse effects and promptly address any issues that arise
• Educate patients on the proper use of antihistamines, including potential side effects and when to seek medical attention
• Coordinate care to ensure continuity and optimize patient outcomes

By working collaboratively and maintaining vigilance, the interprofessional team can help patients achieve the desired therapeutic benefits of antihistamine therapy while minimizing the risk of adverse effects.

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.

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.

action and effect – Fenkarol®

08/05/2022

Contents:

1. What are antihistamines
2. Antihistamines 1st generation
3. 2nd generation medicines
4. 3rd generation antihistamines
5. Phencarol® Allergy Antihistamine
6. Features of taking antihistamines for allergies

An allergic reaction is a common occurrence not only in adults, but also in children, and it is quite difficult to cope with the symptoms on your own. For allergic rhinitis, conjunctivitis, and other disorders that occur against the background of allergies, doctors prescribe antihistamines. Medicines not only stop the symptoms, but also affect the pathogen, suppressing its activity. Any medication for an allergic reaction should be prescribed by a doctor, taking into account the type of allergen, the degree of its effect, the age, and condition of the patient. From the article you will learn what antihistamines are in simple words, get acquainted with their varieties and mechanism of action.

What are antihistamines

Allergy is a chronic disease in which the body reacts undesirably to certain substances. An allergic reaction can occur after an insect bite, inhalation of pollen, touching irritable plants, exposure to household chemicals, etc. In most cases, antihistamine medications are prescribed to eliminate symptoms.

Few people know what an antihistamine is and how it affects the body. The action of the active components of antihistamine drugs is aimed at blocking the production of histamine, a special substance that takes part in the appearance of inflammatory processes, allergic and immune reactions. Each person has a small amount of histamine, which provides the immune system with a natural fight against viruses and bacteria. Allergy releases the substance and causes allergic symptoms.¹

The main task of antihistamines is to block the release of histamine or its receptors. Despite the fact that the allergen enters the body, the release of histamine is not observed, as well as a pathological reaction. When taking an antihistamine, it is possible to get rid of such unpleasant symptoms:

• – stop itching;
• – reduce capillary permeability;
• – relieve swelling in case of allergic inflammation;
• – eliminate the manifestations of rhinitis, conjunctivitis.

Picture 1.1. Drugs help to cope with skin rashes with allergies.

Local and systemic antihistamines are prescribed for allergic rashes

An antihistamine is a mandatory measure in the treatment of an allergic reaction of varying degrees of manifestation. In the absence of therapy in the human body, certain biological substances are produced under the action of the pathogen. The neglected course of allergies leads to convulsions, Quincke’s edema, respiratory rhythm disturbance, bronchospasm, difficult heart function, loss of consciousness and other complications that pose a danger to human health and life.

1st generation antihistamines

This antihistamine is the oldest development that has not lost popularity due to its high efficiency. First generation drugs were developed in the first half of the 20th century. Their main feature is a quick effect on the body and effective elimination of the symptoms of an allergic reaction. The disadvantages of the remedy include a short-term effect, as a result of which it is necessary to take pills often. On average, the duration of action lasts from 4 to 12 hours, depending on the type of drug. The short-term effect is due to the rapid metabolism of the active components and their excretion in the urine. ²

First generation antihistamine is prescribed by a doctor because it has many side effects:

• — weakness throughout the body;
• – increased drowsiness;
• – dizziness and headaches;
• – violation of coordination;
• – decreased concentration and attention;
• – disorders of the psychomotor type;
• – increased eye pressure;
• Difficulty urinating.

The occurrence of adverse reactions is associated with the penetration of active ingredients through the blood-brain barrier, after which they bind to brain receptors and blockade of the central serotonin and M-cholinergic receptors occurs. Such a reaction can provoke a sedative effect of varying severity. After taking this type of antihistamine, a person may experience dry mouth, blurred vision. If adverse reactions occur, treatment with the drug should be discontinued and a doctor should be consulted to prescribe a new one.

The most popular first-generation drugs include Dimedrol, Mebhydrolin, Chloropyramine. Medicines are used for allergic conjunctivitis, rhinitis, the appearance of rashes on the skin. In severe allergies, medications are prescribed in the form of injections, which have a stronger effect and act faster.

2nd generation drugs

People who do not know what histamines are may not know the difference between drugs of different generations. Means of the 2nd generation are more modern and improved. Their main advantage is the minimal effect on serotonin and M-cholinergic receptors, which reduces the likelihood of adverse reactions.

Image 1.2. This group of medicines acts for a longer time, which allows them to be taken less frequently than 1st generation drugs.

The effect after taking 2nd generation antihistamines can last up to 48 hours

These histamines are drugs that affect the body, including the central nervous, immune systems. Second-generation drugs are characterized by minimal or no sedative effect, and there is no tachyphylaxis effect. Thus, long-term treatment with the use of these groups of drugs is allowed, but under the supervision of a doctor and constant monitoring of the work of the heart muscle. These funds include “Loratadin”, Cetirizine, “Ebastine” and many others.

There are also new developments that practically do not have a negative effect on the heart, nervous system, do not suppress mental reactions and are not addictive. The main components of these drugs are fexofenadine and desloratadine. Some are indicated for a seasonal allergic reaction, others are recommended for a chronic form of the disease. The drug begins to act within an hour after ingestion, and the effect can persist throughout the day. The drug blocks the release of histamine, prevents spasm, reduces the manifestation of edema, itching.³

3rd generation antihistamines

This is a modern development that is an improved 2nd generation. Modern antiallergic medicines practically do not have adverse reactions, they differ in a more pronounced and lasting effect. Remedies for an allergic reaction of the 3rd generation do not cause drowsiness, while retaining their effect for 24 hours. The main active ingredients are levocetirizine, desloratadine and fexofenadine. ⁴

Fencarol® Allergy Antihistamine

Phencarol® is used on the recommendation of a physician in the treatment of various types of allergic reactions. A pronounced antihistamine effect allows you to cope with such manifestations of allergies:

• – itching and rashes on the skin;
• – rashes on the body of different localization;
• – allergic rhinitis;
• – inflammation, redness of the mucous eyes.

Image 1.3. Phenkarol® combines the important properties of the 1st and 2nd generation anti-allergy drug. ^5.6

Phencarol® acts on histamine and serotonin receptors and removes histamine from the body . ⁶ Many antihistamines that affect the psycho-emotional state are not allowed to be used during work that requires attention and concentration. The drug Fenkarol® does not have a similar effect, does not enhance the effects of alcohol, and in no way has a negative impact on everyday habitual life. ⁶ It is recommended to check the absence of drowsiness by short-term administration.

With caution, the drug is required to be taken by people with atrial fibrillation, as well as in the presence of other diseases of the cardiovascular system of a severe course. Also, with special attention to taking the drug should be taken to patients with diseases of the gastrointestinal tract, kidneys and liver. Since the drug is not addictive, an increase in dosage during long-term treatment is not required. ⁵

Features of taking antihistamines for allergies

Few people know what a histamine means and the features of its use in treatment. Uncontrolled intake of such drugs, their incorrect dosage can lead to deterioration and complications. Antihistamine drugs can enhance the effect of other drugs, which must be taken into account in complex therapy using several different medications. Especially carefully combine antihistamines with drugs that have an effect on the central nervous system. An increased reaction can lead to an overdose, including loss of consciousness. You should also not combine treatment with antihistamines with alcohol, which can provoke unforeseen reactions and human behavior.

Medicines that have an antihistamine effect also often lead to dryness of the mucous membranes of the mouth and eyes, so additional drugs may be required to eliminate these manifestations. Despite the fact that many medicines have a similar mechanism of action, a completely different drug is suitable for each person. For this reason, you should not self-medicate, but you need to consult a doctor who will select the appropriate remedy, taking into account the individual characteristics of the body of each patient. Knowing what antihistamines help with, how they act on the body, in what quantity they are allowed, patients will be able to avoid unpleasant consequences if the allergy is not properly treated.

1. Clinical pharmacology: a textbook for universities / Ed. V.G. Kukesa. – 4th edition., Revised. and additional, – 2009. – 1056 p.
2. Zaitseva O.V. Antihistamines in pediatric practice: a guide for doctors / O.V. Zaitsev. – Moscow, 2012. – 42 p. 3
3. Gushchin I. S. Antihistamines. A guide for doctors. M.: Aventis Pharma, 2000, 55 p.
4. Clinical pharmacology / ed. V.G. Kukes. VC. – M.: GEOTARMedia, 2013. – 1052 p.
5. Luss L.V. et al. Antihistamines in general clinical practice. Questions and answers. To help the practical doctor. Toolkit. UDC 616-056.3:615.03. 2017 184s.
6. IMP Fenkarol tab. 50mg from 08/05/2021

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Antihistamines for the prevention and treatment of motion sickness

What is the purpose of this review?

Motion sickness, also commonly known as motion sickness or motion sickness, is a collection of symptoms, usually nausea and vomiting. These symptoms are caused by passive body movement, where your body moves without you consciously causing it to move, in response to actual movement (for example, when riding in a car or being in a boat), or the illusion of movement when exposed to virtual movement (for example, virtual reality simulation) and a moving visual environment (for example, when looking out the window of a moving train). Antihistamines are a type of medication commonly prescribed to people to treat or prevent motion sickness. In this study, we wanted to find out if these drugs really work to achieve this goal.

Key information

We have found that antihistamines are likely to actually reduce the risk of motion sickness symptoms in natural driving situations (eg, ship or aircraft) compared to placebo (dummy) in adults prone to motion sickness. We also found that antihistamines were more likely to cause drowsiness than placebo. We did not find any studies examining whether antihistamines are effective in treating motion sickness once it has started, and there is very little information on their effect in children under 18 years of age. For all other outcomes that have been studied, there is uncertainty about the true effect of antihistamines compared to other drugs and non-drugs, or other side effects and effects on bodily functions (eg, heart rate or stomach movements).

What was learned in this review?

We looked at studies in which people known to have motion sickness were given an antihistamine or a placebo (a sham treatment). We also looked at those who were prescribed antihistamines versus other drugs or other non-drug therapies.

What are the main findings of this review?

Antihistamines versus placebo

The results indicate that antihistamines are probably more effective than placebo in preventing symptoms of motion sickness in vivo.

There is uncertainty about whether antihistamines are effective in preventing motion sickness or whether they affect gastric tachyarrhythmia (the way the inside of your stomach moves) under experimental (laboratory) conditions compared to placebo.

Antihistamines may cause more sedation (drowsiness) than placebo. There is uncertainty about whether antihistamines cause blurred vision (inability to see clearly) or cognitive impairment (inability to think clearly) compared to placebo.

Antihistamines versus scopolamine

There is uncertainty about the effectiveness of antihistamines in preventing motion sickness or their ability to induce drowsiness compared to scopolamine in vivo.

Antihistamines versus antiemetics

Uncertainty exists regarding the effectiveness of antihistamines in preventing motion sickness or their ability to induce drowsiness compared to natural antiemetics.

Antihistamines versus acupuncture

There is uncertainty about the effectiveness of antihistamines in preventing motion sickness compared to acupuncture in vitro.