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

Preludin limits appetite because of its effects on the hypothalamus, a control center in the brain. The typical dose is 75 mg once a day.

Preludin is 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 the active ingredient in Bontril and Plegine, phendimetrazine. Others include the active ingredients in the over-the-counter products, Acutrim and Dexatrim, and in the prescription medications: Tenuate, Mazanor, Sanorex, Bontril, Plegine, Adipost, Dital, Dyrexan, Melfiat, Prelu-2, Rexigen Forte, Fastin, Ionamin, and Didrex.

Possible side effects of Preludin 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.