Belviq works by activating an area of your brain called the serotonin 2C receptor. When this receptor is activated, it can make you feel full faster so that you stop eating sooner and eat less than usual. Fenfluramine worked by activating a slightly different receptor, called serotonin 2B. When Belviq is taken at the recommended dosage of 10 milligrams (mg) twice a day, it does not appear to activate the serotonin 2B receptor or to cause heart valve abnormalities.
Phentermine, the other drug in Fen-Phen, is also an appetite suppressant and is still on the market. It's meant to be used short term to promote weight loss in people who are very overweight and who have a weight-related condition, such as high blood pressure or type 2 diabetes.