Antihistamine
A medicine that blocks histamine, the chemical released in allergic reactions.
What it means
An antihistamine blocks histamine, the chemical released by mast cells during an allergic reaction, which produces itching, sneezing, runny nose, watery eyes and hives.
Two generations:
- Non-sedating (cetirizine, loratadine, fexofenadine) — do not readily cross into the brain. First choice for essentially all allergy.
- Sedating (chlorphenamine, promethazine) — cause marked drowsiness and impair concentration well into the next day.
In practice
H2 blockers such as famotidine act on a different histamine receptor and reduce stomach acid rather than treating allergy — same word, different job.
Sedating antihistamines are widely sold as sleep aids. They are not recommended for insomnia: tolerance develops quickly and next-day impairment is common.
Why it matters
For hay fever, a steroid nasal spray is more effective than any antihistamine for blocked nose and should be started before the season, used daily, and given two weeks to work.
Antihistamines do not treat anaphylaxis. Adrenaline does. Giving an antihistamine while waiting is not treatment — anaphylaxis means adrenaline and 999.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026