Phentermine first received approval from the Food and Drug Administration (FDA) in 1959 as an appetite suppressant for the short-term treatment of obesity. Phentermine is the most commonly prescribed prescription appetite suppressant, accounting for 50% of the prescriptions. Part of this reason is because it's significantly cheaper than the other major FDA-approved diet drugs, Meridia and Xenical.

Phentermine resin became available in the United States in 1959 and Phentermine Hydrochloride in the early 1970s. In the US , Phentermine is currently sold under the brand names Ionamin® (Medeva Pharmaceuticals) and Adipex-P® (Gate Pharmaceuticals). It is also available as a generic medication, known as 'phentermine'.

As Phentermine is an older drug, no new efficacy trials have been conducted. The one notable exception, are several trials on the combination of Phentermine and Fenfluramine in the early and mid 1990s.

Fen-Phen (or Phen-Fen) and Dexfen-Phen - two passing trends
Fen-Phen refers to the combination, or cocktail, of Fenfluramine or Pondimin (the "Fen") and Phentermine (the "Phen"). Fenfluramine received FDA approval in 1973 for the short-term treatment of obesity. The FDA never approved the Fen-Phen combination, but once the agency has approved a drug, doctors may prescribe it at will. Their use, together, was considered "off-label". Online Phentermine

Together, Phentermine and Fenfluramine produced a powerful diet drug cocktail.

The Fen-Phen cocktail became an overnight sensation. 1996 saw 6.6 million prescriptions of Fen-Phen in the U.S. Dexfen-Phen refers to the combination, or cocktail, of Dexfenfluramine or Redux (the "Dexfen") and Phentermine (the "Phen"). Dexfenfluramine received FDA approved in 1996 for use as an appetite suppressant in the management of obesity. Like Fen-Phen, Dexfen-Phen, too, became an overnight sensation. Neither combination, however, was ever tested for safety.

Phentermine is used as an adjunct to exercise, behavioral modification, and caloric restriction in the short-term management (a few weeks) of exogenous obesity. Phentermine therapy is indicated for patients with no underlying risk factor but a pretreatment body mass index (BMI) of 30 kg/m 2 or greater and those with an underlying risk factor (e.g., hypertension, diabetes mellitus, hyperlipidemia) and a pretreatment BMI of 27 kg/m 2 or greater. Phentermine is indicated only for monotherapy in the management of exogenous obesity; the drug should not be used in combination with selective serotonin-reuptake inhibitor antidepressants (e.g., fluoxetine, fluvoxamine, paroxetine, sertraline) or monoamine oxidase (MAO) inhibitors. Phentermine To help bring about and maintain loss of weight, the patient must be prepared to curtail overeating and to consume a suitable diet. Phentermine also has been used for longer periods combined with fenfluramine (no longer commercially available in the US ) in selected patients for the management of this condition. Such combined long-term therapy had been used widely in the 1990s in the management of exogenous obesity. However, because of accumulated data on adverse effects associated with the drugs, fenfluramine hydrochloride (Pondimin ® ) and its dextrorotatory isomer dexfenfluramine hydrochloride (Redux ® ) were withdrawn from the US market in 1997.