Phentermine is a phenethylamine, more importantly a derivative of amphetamine, primarily used as an appetite suppressant. It is typically prescribed for individuals who are at increased medical risk because of their weight, as opposed to cosmetic weight loss. Phentermine is sold either as an immediate-release formulation (Adipex?®) or as a slow-release resin (Ionamin?®, Duromine?® in Australia and New Zealand).