How Do Weight Loss Medications work?

Ads for diet pills are everywhere. Some are for weight loss drugs that the U.S. Food and Drug Administration (FDA) has pproved as safe and effective. Others are for dietary supplements that are not subject to FDA approval at all. These products, and their promises of effortless weight loss, may seem to be a dream come true. How do they work?

The FDA has approved three prescription weight-loss drugs. One is orlistat (Xenical); the over-the-counter drug Alli is a low-dose version of the same medicine. This drug interferes with lipase, the enzyme that digests fat in the small intestine. Undigested fat leaves the body in feces; orlistat therefore reduces calorie intake by reducing the body’s absorption of high-energy fat molecules. The other two prescription weight-loss drugs are sibutramine (Meridia) and phentermine (Adipex- P). These medicines also reduce calorie intake, but in a different way: They suppress appetite.

All three prescription drugs can help a person lose weight but only if combined with exercise, a low-calorie diet, and behavior modification. Each also has side effects.

Dietary supplements greatly outnumber prescription weight-loss drugs. The FDA does not require the manufacturers of dietary supplements to show that the remedies are either safe or effective. Ads for “natural” supplements such as hoodia, green tea extract, and fucoxanthin make extraordinary promises of rapid weight loss, but the claims remain largely untested in scientific studies. The mechanism by which they work (if they work at all) usually remains unclear.

Unfortunately, some dietary supplements have serious side effects. Ephedra is one example. Before 2004, ephedra was marketed as a weight-loss aid and energy booster, but studies eventually linked it to fatal seizures, strokes, and heart attacks. The FDA therefore banned the sale of ephedra in the United States in 2004. An herb called bitter orange has taken its place in many “ephedra-free” weight-loss aids. But bitter orange has side effects that are similar to ephedra’s, and its safety remains unknown.

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