Promethazine is a drug used in the treatment of the allergic response, such as combating hay fever. Promethazine is also used in the treatment of insomnia, and nausea and vomiting (for patients in the post-surgery period). Promethazine belongs to the phenothiazine class of drugs and is also classified as a first-generation antihistamine.
As such, promethazine has a diverse range of effects. It blocks histamine receptors (which combats the allergic response) but also has potent sedative effects. Promethazine also has antipsychotic properties. It also acts via dopamine receptors to help patients with nausea.
Common side effects with promethazine include drowsiness, dizziness, double vision, anxiety, fatigue, and dry mouth.