Drugs that combat the histamine released during an allergic reaction by blocking the action of the histamine on the tissue.

Antihistamines do not stop the formation of histamine; therefore, antihistamines do not stop the allergic reaction but protect tissues from some of its effects.

Antihistamines frequently cause mouth dryness and sleepiness. Newer "non sedating" antihistamines are generally thought to be somewhat less effective.

Antihistamine side effects that occasionally occur include urine retention in males and fast heart rate.