antihistamine

An antihistamine takes the sting out of an allergic reaction by silencing histamine, the chemical the body floods out when it meets an allergen. Histamine is what makes you sneeze, itch, run and swell; block its receptor and those symptoms fade.

In everyday use 'antihistamine' means an H1 receptor antagonist, which blocks histamine's H1 receptor to relieve itching, sneezing, runny nose, watery eyes and hives. The older 'first-generation' drugs (diphenhydramine, chlorphenamine) cross into the brain and cause drowsiness, while 'second-generation' drugs (cetirizine, loratadine, fexofenadine) are designed to stay out of the brain, so they are far less sedating and preferred for daytime allergy control. Strictly, they act as inverse agonists, shifting the receptor toward its inactive state.

Antihistamines treat allergy symptoms but do nothing for the airway-muscle spasm of an asthma attack, and they are not the same as the H2 blockers used for stomach acid. The sedating older drugs also have antimuscarinic effects — dry mouth, blurred vision, urinary retention — and are riskier in the elderly, which is another reason the non-drowsy newer agents are usually chosen.