Phentermine What is phentermine? Phentermine is a stimulant that is similar to an amphetamine. Phentermine is an appetite suppressant that affects the central nervous system. Phentermine is used together with diet and exercise to treat obesity (overweight) in people with risk factors such as high blood pressure, high cholesterol, or diabetes. Phentermine may also be used for other purposes not listed in this medication guide. Important information about phentermine Do not use phentermine if you haveused an MAO inhibitor such as furazolidone (Furoxone), isocarboxazid (Marplan), phenelzine (Nardil), rasagiline Azilect), selegiline (Eldepryl, Emsam, Zelapar), or tranylcypromine (Parnate) in the last 14 days. A dangerous drug interaction could occur, leading to serious side effects. Taking phentermine together with other diet medications such as fenfluramine (Phen-Fen) or dexfenfluramine (Redux) can cause a rare fatal lung disorder called pulmonary hypertension. Do not take phentermine with any other diet medications without your doctor's advice. Video: Diabetes Why managing blood sugar is so important. Phentermine may impair your thinking or reactions. Be careful if you drive or do anything that requires you to be alert. Drinking alcohol can increase certain side effects of this medicine. Weight loss during pregnancy can harm an unborn baby, even if you are overweight. Do not use phentermine if you are pregnant. Phentermine is only part of a complete program of treatment that may also include diet, exercise, and weight control. Follow your diet, medication, and exercise routines very closely. Phentermine may be habit-forming and should be used only by the