Phentermine is used for a limited period of time to accelerate weight loss in overweight people who exercise and follow a low calorie diet. Phentermine belongs to a class of medications called anorectics. It works by decreasing appetite.Phentermine is used for a limited period of time to accelerate weight loss in overweight people who exercise and follow a low calorie diet. Phentermine belongs to a class of medications called anorectics. It works by decreasing appetite.