Promethazine is an antihistamine commonly used to treat allergy symptoms, motion sickness, nausea, and vomiting. It works by blocking histamine receptors in the body, which helps alleviate allergic reactions and provides sedation. Additionally, it can be used as a preoperative sedative and to enhance the effects of other medications. Promethazine is available in various forms, including tablets, syrups, and injections.