Phentermine (phenyl-tertiary-butylamine), under several brand names including Ionamin and Sentis, is a drug used in conjunction with diet and exercise to treat obesity. It is taken by mouth for up to a few weeks at a time, after which the benefits diminish. It is also available as a phentermine/topiramate combination. Phentermine is used for a short time to promote weight loss, if exercise and calorie reduction is not enough, and in addition to exercise and calorie reduction