A drug that blocks the action of histamine, the mediator behind allergy symptoms and, in foods, behind scombroid fish poisoning. First-generation antihistamines relieve sneezing, itching, and hives but cause drowsiness; second-generation versions target peripheral receptors more selectively with less sedation. They are the first-line response to mild food allergic reactions, though they cannot halt anaphylaxis, for which adrenaline is essential. The histamine connection also explains their limits in scombroid poisoning: the illness mimics allergy because spoiled fish contains preformed histamine, and antihistamines help symptoms but prevention through proper fish chilling is the real answer. As with all drugs affecting alertness, the older antihistamines impair driving, a practical concern for anyone self-medicating hay fever.