Promethazine is a prescription medication used to treat allergic reactions and nausea. Promethazine belongs to a group of drugs called antihistamines. It blocks the effect of histamine, a chemical in the body that causes symptoms of allergic reactions. Promethazine also belongs to another group of drugs called antiemetics, which reduce feelings of nausea.

Promethazine comes as a tablet, oral solution (liquid), rectal suppository, and a liquid to be injected into a muscle.  It is usually taken once daily at bedtime or before meals every six to twelve hours as needed to relieve symptoms of allergy or nausea.