Mepergan Fortis

This combination medicine is used to treat moderate to severe pain. Meperidine is a narcotic pain reliever similar to morphine. Promethazine is used to prevent and treat nausea and vomiting. It may also be used to help you feel calmer before/after surgery or to help the meperidine work better.

Meperidine acts on certain centers in the brain to give you pain relief. Promethazine is an antihistamine (phenothiazine type) that works by acting on certain parts of the brain to decrease anxiety and nausea.
ref: http://www.webmd.com