DEXTROMETHORPHAN; PROMETHAZINE is a cough suppressant and an antihistamine. It is used to treat coughing due to colds or allergies. This medicine will not treat an infection. The lowest GoodRx price for the most common version of promethazine DM is around $4.00, 66% off the average retail price of $12.01. Compare antihistamine / non-narcotic antitussive combinations.