Antihistamines are a class of drug used to treat allergy symptoms such as allergic rhinitis. Histamine is a chemical released in the body in response to exposure to allergens. When histamine binds to a receptor, called an H-1 receptor, on certain cells in the body, it causes the classic signs and symptoms of an allergy including runny nose, itchy eyes, cough, or itching. Antihistamines block this response by binding to the H-1 receptor, preventing histamine from initiating the cascade of allergic symptoms. All antihistamines function by binding to the H-1 receptor. Newer generation antihistamines specifically bind to H-1 receptors only. The older antihistamines, called first-generation antihistamines, are non-selective, meaning they bind to other kinds of receptors, accounting for their diverse side effects. Examples of first generation antihistamines include brompheniramine, chlorpheniramine, diphenhydramine, doxylamine, promethazine and hydroxyzine.
Drowsiness
First-generation antihistamines are able to cross the blood-brain barrier into the brain, unlike newer generation antihistamines, and bind to certain receptors in the brain that increase sleepiness, according to FamilyDoctor.org. First-generation antihistamines are used in over-the-counter sleep aids and are effective in inducing sleep. When driving or performing activities that require mental alertness, first-generation antihistamines should not be utilized.
Anti-cholinergic Side Effects
The first-generation antihistamines, along with their effects of H-1 receptors, can also block other receptors in the brain. These non-selctive antihistamines are potent anti-cholinergic drugs and block the action of acetylcholine. The Merck Manual lists several effects of acetylcholine including helping with memory and learning, as well as functioning of the heart, blood vessels and airways. It is also an important chemical that promotes urinary and digestive tract function.
When acetylcholine is blocked, such as by the first-generation antihistamines, numerous side effects can occur. As a result, according to the Oregon State University College of Pharmacy, patients who take first-generation antihistamines may experience confusion, dizziness, lack of coordination, urinary retention, constipation, nausea and fast heart rhythms as well as a dry mouth and drying of respiratory secretions.
Urinary Retention in the Elderly
The elderly have a greater propensity to develop side effects from first-generation antihistamines. The American Academy of Allergy, Asthma and Immunology reports that seniors have a decreased level of acetylcholine at baseline and, thus, any blocking of acetylcholine in the elderly results in more marked anti-cholinergic side effects when compared to younger populations. Urinary retention is particularly prominent and men with prostate enlargement should not use first-generation antihistamines.
References
- FamilyDoctor.org: Antihistamines: Understanding Your OTC Options
- MayoClinic.com: Antihistamine (Oral Route, Parenteral Route, Rectal Route)
- Oregon State University College of Pharmacy: Antihistamines, H1 Receptor Antagonist Review
- The Merck Manual of Health and Aging: What Are Anticholinergic Effects
- American Academy of Allergy, Asthma and Immunology: Seniors and Asthma
Member Comments