Phentermine, a contraction of "phenyl-tertiary-butylamine", is a psychostimulant drug of the phenethylamine class, with pharmacology similar to amphetamine. It is used medically as an appetite suppressant.

It is approved as an appetite suppressant to help reduce weight in obese patients when used short-term and combined with exercise, diet, and behavioral modification. It is typically prescribed for individuals who are at increased medical risk because of their weight and works by helping to release certain chemicals in the brain that control appetite.

Phentermine is used for the short-term treatment of obesity.

Generally, phentermine appears to be relatively well tolerated. It can produce side effects consistent with its catecholamine-releasing properties, e.g., tachycardia (increased heart rate) and elevated blood pressure, but the incidence and magnitude of these appear to be less than with the amphetamines. Because phentermine acts through sympathomimetic pathways, the drug may increase blood pressure and heart rate. It may also cause palpitations, restlessness, and insomnia. Additionally, phentermine has the potential to cause psychological dependence. After short term use, tolerance begins and can be followed by rebound weight gain.