Antihistamines
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 nor do they stop the conflict between the IgE and antigen. 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 very occasionally occur include urine retention in males and fast heart rate.
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Drugs having an action antagonistic to that of histamine on either H1 or H2 receptors; H1 type a. are used in the treatment of allergy symptoms; whereas H2 type a. reduce gastric acidity in ulcer disease and gastroesophageal reflux.

Medical dictionary. 2011.

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