Promethazine is a medication primarily used to treat allergies, motion sickness, and nausea. It belongs to a class of drugs known as antihistamines, which work by blocking the effects of histamine, a substance in the body that causes allergic symptoms. Promethazine can also be used as a sedative to help with sleep. In addition to its antihistamine properties, promethazine is often prescribed to prevent nausea and vomiting related to surgery or chemotherapy. It is available in various forms, including tablets, syrup, and injections, making it versatile for different patient needs.