Phentermine is a prescription appetite suppressant used for the treatment of excess weight and obesity. It works by suppressing the appetite by changing the chemistry in the brain in a way that decreases the hunger drive. Individuals taking these medications lose weight because they are less hungry and thus consume fewer calories. These medications help you feel more in control of your appetite and food cravings. They can increase your satiety, which means that when you eat you will feel full sooner. Phentermine can also help take your mind off food so that you can think about other things. Phentermine was approved by the FDA in 1959 and has been taken by millions of patients.   Fen/fen was a two-drug combination of fenfluramine and Phentermine Fenfluramine is not related chemically to phentermine. Fenfluramine was taken off the market in 1997 because it was thought to cause a serious complication known as pulmonary hypertension in some patients. Phentermine has never been associated with pulmonary hypertension.