Relieving allergy symptoms (eg, itching, hives). It is used to prevent and control nausea and vomiting during and after surgery. It is also used to help produce light sleep; treat motion sickness; or treat pain after surgery, in combination with other medicines. It may also be used for other conditions as determined by your doctor. Promethazine is a phenothiazine antihistamine. It works by blocking histamine, which decreases the symptoms of allergies and helps decrease nausea and vomiting.