Antihistamines: Overview and Common Types

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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.