Phentermine is an appetite suppressant, which affects the central nervous system and helps users eat less. Phentermine is most effective when used in conjunction with a special diet and exercise course. This drug is mainly prescribed to patients with Body Mass Index (BMI) of 28 or more, or those diagnosed with health risks related to obesity. Other criteria for phentermine use are also normal blood pressure, not pregnant or nursing, ages between 18 and 60, and no foreseeable medical conditions.

Phentermine is available in two forms: tablets (resin) and extended-release capsules (hydrochloride). Phentermine hydrochloride is a bitter, white, odorless salt that is soluble in water and alcohol. Adipex-P is an immediate release version of the hydrochloride. Phentermine resin is a course granular substance that is almost insoluble in water, with Ionamin being the resin compounds that allows the medicine to be released slowly into the body. The capsules are generally taken in a single dose in the morning, and the tablets are taken three times a day, thirty minutes prior to a meal. The recommended period of use is from six to twelve weeks.