Promethazine HCl for Injection 50 mg per mL Ampules 1 mL x 25-Pack (Rx)Promethazine HCl for Injection 50 mg mL Ampules 1 mL x 25 Tray. Promethazine hydrochloride is a phenothiazine derivative which possesses antihistaminic, sedative, antimotion sickness, antiemetic, and anticholinergic effects. Promethazine is a competitive H1 receptor antagonist, but does not block the release of histamine. Structural differences from the neuroleptic phenothiazines result in its relative lack (1 10 that of chlorpromazine) of dopamine