Antihistamines: Overview and Common Types
Antihistamines are a class of drugs that block the action of histamine, a chemical involved in allergic reactions, gastric acid secretion, and neurotransmission. They are used to treat allergies, hay fever, hives, anaphylaxis (emergency), motion sickness, insomnia, and gastric acid-related conditions.
How Antihistamines Work
Antihistamines block histamine receptors in the body. The main receptor types are:
- H1 receptors – Found in smooth muscle, endothelium, and CNS; blocking relieves allergies, hives, and can cause sedation.
- H2 receptors – In stomach lining; blocking reduces gastric acid.
- H3 receptors – In CNS; regulate histamine release and wakefulness.
- H4 receptors – In bone marrow and white blood cells; involved in immune responses.
Common H1 Antihistamines
- Diphenhydramine – First-generation, sedating, used for allergies and insomnia
- Chlorpheniramine – First-generation, mild sedation, allergy treatment
- Promethazine – First-generation, sedating, used for allergies, nausea, and motion sickness
- Loratadine – Second-generation, non-sedating, long-acting
- Cetirizine – Second-generation, slightly sedating, fast onset
- Fexofenadine – Second-generation, non-sedating, seasonal allergies
Common H2 Antihistamines
- Ranitidine (less used now due to recalls)
- Famotidine
- Nizatidine
- Cimetidine
Common H3 Antihistamines
- Pitolisant (used in narcolepsy)
- Others mostly experimental or CNS-targeted
Common H4 Antihistamines
- Mostly in research/trials for inflammation or autoimmune conditions
Tips & Notes
- First-generation H1 antihistamines often cause drowsiness, dry mouth, and other anticholinergic effects.
- Second-generation H1 antihistamines are preferred for daytime allergy treatment due to minimal sedation.
- H2 blockers are mainly for acid reflux or ulcer prevention.
- Avoid mixing sedating H1 antihistamines with alcohol or CNS depressants.
This post is intended for educational discussion and harm reduction. Always consult a healthcare professional before starting any medication.