When used as directed in a comprehensive, physician-supervised weight-loss program that includes a low-fat diet and regular exercise, these drugs can increase weight loss by about an additional 10%. However, weight loss may not be permanent, especially after they are discontinued.

These medications limit appetite because of their effects on the hypothalamus, a control center in the brain. The typical dose is 35 mg two or three times a day, one hour before meals.

These drugs are chemically related to amphetamine. Years ago, amphetamine was prescribed for weight loss because it is an appetite suppressant. But it often led to drug abuse, so pharmacologists tinkered with its molecular structure to develop similar -- but safer -- compounds. One is phendimetrazine, the active ingredient in these drugs. Others include the active ingredients in the over-the-counter products, Acutrim and Dexatrim, and in the prescription medications Tenuate, Mazanor, Sanorex, Preludin, Fastin, Ionamin, and Didrex.

Possible side effects of these drugs include nervousness, irritability, headache, sweating, dry mouth, nausea, and constipation. Do not take this drug if you are also taking any antidepressant in the chemical family called monoamine oxidase (MAO) inhibitors.