Promethazine-d3 (hydrochloride) is deuterium labeled Promethazine (hydrochloride). Promethazine hydrochloride is an orally active phenothiazine derivative with antihistaminic (H1), sedative, antiemetic, anticholinergic, and antimotion sickness properties. Promethazine hydrochloride is a potent H1 receptor antagonist and a mAChR antagonist. It also has a certain affinity for 5-HT2A and 5-HT2C receptors[1][2].