Drug class · Skin
Antihistamines
Also known as: H1 antagonists, allergy tablets
Medicines that block histamine's H1 receptors to relieve hay fever, hives and itching. Newer members (cetirizine, loratadine, fexofenadine) cause little drowsiness; older ones (chlorphenamine, promethazine) sedate.
- Biological target
- Histamine H1 receptor
- Also called
- H1 antagonists, allergy tablets
Mechanism of action
When mast cells release histamine in an allergic reaction, it makes capillaries leak (swelling, runny nose), stimulates itch nerves and constricts airways. Antihistamines occupy the H1 receptors so these effects are muted. Second-generation drugs cross the blood-brain barrier poorly, which is why they are much less sedating. They are adjuncts only in anaphylaxis, where adrenaline is the treatment.
Members of the class
Members with a page on this site are linked.
Conditions treated
Class effects
- Drowsiness (marked with older agents), dry mouth, headache
Cautions
- Check driving response; avoid alcohol
- Older agents cause confusion and falls in the elderly and urinary retention with prostate enlargement
- Not a substitute for adrenaline in anaphylaxis
Class warnings
- Sedating (first-generation) antihistamines cause drowsiness, impair driving and interact with alcohol and other sedatives; non-sedating antihistamines are preferred for daytime use and for older people. UK
Class interactions
| With | Effect | Source |
|---|---|---|
| Alcohol, opioids, benzodiazepines and other sedatives (sedating antihistamines) | Additive drowsiness and impaired coordination. | UK |
| Antimuscarinic drugs (sedating antihistamines) | Additive dry mouth, constipation, urinary retention and confusion in older people. | UK |
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Educational content. A drug class page describes what a family of medicines has in common. Individual medicines differ in dose, licensing and interactions; treatment choices belong to a prescriber.