An exogenous agent that inhibits the release of histamine, the amino acid derivative that stimulates vasodilation and permeability under many circumstances, particularly tissue irritation. The most common type of antihistamine, the H1 receptor antagonist, produces many moderate side effects, and the H2 receptor antagonist cimetidine is even more problematic. That they are so commonly used can lull both physician and patient into trivializing their iatrogenic potential. Histamines, which are most abundant in the skin, respiratory, and GI tract mucus membranes, help heal; using antihistamines to inhibit the healing response for the whole body simply in order to lessen the acute but physiologically superficial symptoms of something like hay fever is to risk many subtle side effects.