Generic Phenergan belongs to a group of drugs called phenothiazines. It works by changing the actions of chemicals in your brain. Promethazine (the active ingredient) also acts as an antihistamine. It blocks the effects of the naturally occurring chemical histamine in your body. Phenergan is used to treat allergy symptoms such as itching, runny nose, sneezing, itchy or watery eyes, hives, and itchy skin rashes. Phenergan also prevents motion sickness, and treats nausea and vomiting or pain after surgery. It is also used as a sedative or sleep aid.