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Promethazine is an H1 receptor blocking agent. In addition to its antihistaminic action, it provides clinically useful sedative and antiemetic effects. Promethazine is well absorbe from the gastrointestinal tract. Clinical effects are apparent within 20 minutes after oral administration and generally last six to 12 hours. Promethazine is metabolized by the liver to a variety of compounds; the sulfoxides of promethazine and N-demethylpromethazine are the predominant metabolites appearing in the urine.