The hydrochloride salt form of promethazine, a phenothiazine derivative with antihistaminic, sedative and antiemetic properties. Promethazine hydrochloride selectively blocks peripheral H1 receptors thereby diminishing the effects of histamine on effector cells. Promethazine hydrochloride also blocks the central histaminergic receptors, thereby depressing the reticular system causing sedative and hypnotic effects. In addition, promethazine hydrochloride also has centrally acting anticholinergic properties and probably mediates nausea and vomiting by acting on the medullary chemoreceptive trigger zone.