Xenical (orlistat) is a fat blocker that prevents fat from being completely absorbed in the gastrointestinal tract. Prescription strength is 120 milligrams; over-the-counter Alli is the same drug at half strength, or 60 milligrams.

Meridia (sibutramine) helps reduce hunger by working on the appetite control center in the brain that makes you feel full. "Meridia can help add 5%-10% additive weight loss that you would not get with diet and exercise alone" says Aronne.

Phentermine is generic, the least expensive of the medications, and has been on the market the longest. Once a part of the famed fen-phen combination weight loss drug, Phentermine works to decrease appetite. It is the most widely used weight loss drug.