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Phentermine
(Brand names: Adipex, Ionamin, or Fastin)

How Phentermine works

Phentermine (Brand names: Adipex or Ionamin) is in a class of medicines called sympathomimetics. This is the same class that the weight loss drug sibutramine (Meridia or Reductil) is in. They work by increasing the activation of a branch of the nervous system called the sympathetic nervous system. One of the effects of activating the sympathetic nervous system is a decrease in appetite. That is, people are less hungry on the medicines.

Pros

When phentermine (brand names Adipex-P or Ionamin) is combined with healthy lifestyle changes, studies have shown that people lose weight.

Cons

It is only approved for short term use. This is generally considered to be about 12 weeks. When most people stop using diet pills, they gain back the weight.

There are several side effects to this class of drugs. High blood pressure and increased heart rate are the most significant. Others include dry mouth, nervousness, constipation, and insomnia. Phentermine is a schedule IV prescription drug. This means there are more restrictions on its use.

Phentermine used to be commonly combined with fenfluramine. It was called "phen-fen," and this combination was associated with heart valve abnormalities. The FDA had it withdrawn from the market in 1997.

With continued use, your body can become tolerant to the drug, and then there is an increased risk of dependence.

Other Comments

This weight loss medicine does not replace diet and exercise. You should thoroughly discuss the risks and benefits of phentermine with your doctor before using it.

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It is important you discuss any weight loss or exercise plan with your doctor. Only you and your physician can decide what is best for you. Some people have certain conditions that prevent them from doing all exercises, and goal body weights may be different for different people. You need to discuss all these things with your physician before starting any weight loss or exercise program.

This article was written by John Vickery, MD.

References
Ann Intern Med 2005;142:525-531
“Drug therapy of Obesity” Up To Date version 15.3
Treatment Guidelines from the Medical Letter 2008;6:23-28