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

These phentermine drugs, nicknamed "phen," were often prescribed along with Pondimin (fenfluramine) or Redux (dexfenfluramine), nicknamed "fen," in combinations known as "fen-phen." But in September 1997, the Food and Drug Administration asked manufacturers to withdraw Pondimin and Redux because of studies strongly suggesting that the drugs damage the heart's mitral valve. Today, the "fen" drugs are no longer available, but the "phen" medications still are.

Fastin and Ionamin limit appetite because of their effects on the hypothalamus, a control center in the brain. The typical dose is 8 mg three times a day, 30 minutes before meals, or 15 to 37.5 mg a day before breakfast.

Fastin and Ionamin are chemically related to amphetamine. Years ago, amphetamine was prescribed for weight loss because it is an appetite supprressant. But it often led to drug abuse, so pharmacologists tinkered with its molecular structure to develop similar -- but safer -- compounds. One is phentermine, the active ingredient in Fastin and Ionamin. Others include the active ingredients in the over-the-counter products Acutrim and Dexatrim, and in the prescription medications Tenuate, Mazanor, Sanorex, Preludin, Bontril, Plegine, Adipost, Dital, Dyrexan, Melfiat, Prelu-2, Rexigen Forte, and Didrex.

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