Phentermine for obesity


Generic Name Brand Name
phentermine Adipex-P, Ionamin

How It Works

Phentermine suppresses your appetite so that you feel less hungry. It works by changing levels of brain chemicals (neurotransmitters) that affect mood and appetite. It may also slightly increase the rate at which your body burns calories.

Why It Is Used

Appetite suppressant medicines such as phentermine help people who are obese (those with a body mass index of 30 or higher) to lose weight. They are generally used with a weight-loss diet and exercise program.

How Well It Works

Weight loss varies depending on how long the medicine is taken. A review of research reports that using 15 to 30 mg of phentermine daily resulted in an average weight loss of about 8 lb (3.6 kg) more than when taking a placebo. The medicine was used from 2 weeks to 24 weeks, with an average use of about 13 weeks. In the studies reviewed, most of the people using phentermine also made lifestyle changes, such as diet or exercise. 1, 2

Side Effects

Side effects of phentermine include:

  • Nervousness.
  • Irritability.
  • Sweating.
  • Nausea.
  • Headache.
  • Dry mouth.
  • Constipation.
  • Sleep problems.
  • Possible addiction.

Most of these side effects are mild, and they usually improve with continued treatment.

See Drug Reference for a full list of side effects. (Drug Reference is not available in all systems.)

What To Think About

Phentermine is no longer marketed in Europe due to a possible association with heart and lung problems.1

It is not known whether these medicines are safe to take for more than a few months. So phentermine is approved for short-term use only.3

Weight generally is regained after stopping medicine.

The best weight-loss medicine program will also incorporate a weight-loss diet and exercise program.

Complete the new medication information form (PDF)(What is a PDF document?) to help you understand this medication.



  1. Arterburn DE, et al. (2008). Obesity in adults, search date February 2007. Online version of BMJ Clinical Evidence:
  2. Li Z, et al. (2005). Meta-analysis: Pharmacologic treatment of obesity. Annals of Internal Medicine, 142(7): 532–546.
  3. Purnell JQ (2008). Obesity. In DC Dale, DD Federman, eds., ACP Medicine, section 3, chap. 10. Hamilton, ON: BC Decker.

Last Updated: April 16, 2009

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