Antihistamines such as promethazine compete with histamine for one of the receptors for histamine (the H1 receptor) on cells. However, when the antihistamines bind to the receptors they do not stimulate the cells. Instead, they prevent histamine from binding and stimulating the cells. Promethazine also blocks the action of acetylcholine (anticholinergic effect), and this may explain its benefit in reducing the nausea of motion sickness. It is used as a sedative because it causes drowsiness as a side effect. The FDA approved promethazine in 1951.
HOW TO USE: May be taken with food or milk to lessen stomach irritation. Do not take more medication than is prescribed. For motion sickness, medication should be taken 30-60 minutes before travel.
Promethazine is a phenothiazine derivative that competitively blocks histamine H1 receptors without blocking the secretion of histamine. It also is a very weak dopamine antagonist
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