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.

Fat blockers reduce fat absorption and, as a result, some people are plagued with oily discharge, fatty stools, gas, and/or inability to control bowel movements. These side effects are the result of the fat not being absorbed by the body. Eating a low-fat diet reduces the risk of potential side effects. But if your diet is already low in fat, you may see less of an impact from the medication, as there is less fat to block.

Xenical or Alli is safe for almost anyone because the drug is not absorbed, says Kushner. "I frequently prescribed it to cardiovascular patients because it is so safe," he says.

Patients must be willing to stick to a low-fat diet. These drugs have a built-in feedback system. "If you eat a high-fat diet, you will experience the side effects, so to avoid the unpleasant effects, you need to reduce the fat in your diet," says Aronne.

Not only do these drugs block fat, they have the potential to block other nutrients.

"Take a once-daily multivitamin mineral for nutritional insurance to help compensate for any losses and don't take it at the same time as the medication," says Dawn Jackson-Blatner, a spokesperson for the American Dietetic Association.