DEXTROMETHORPHAN; promethazine could be a cough appetite suppressant and an antihistamine. it’s used to treat coughing because of colds or allergies. This drugs won’t treat an infection. the lowest GoodRx price for the most common version of promethazine DM is around $4.00, sixty six off the typical retail value of $12.01. Compare medication / non-narcotic antitussive mixtures.