Because of risk of severe tissue injury, including gangrene requiring amputation, following IV administration of promethazine, FDA states that the preferred route of administration is deep IM injection.

Perivascular extravasation, unintentional intra-arterial injection, and intraneuronal or perineuronal infiltration of promethazine may result in irritation and tissue damage. Healthcare professionals should be alert for signs and symptoms of potential tissue injury, including burning or pain at the site of injection, phlebitis, swelling, and blistering. (See IV Administration under Dosage and Administration and see Precautions Associated with Parenteral Administration under Cautions.)