Prescription Diet Pills

Different diet pills work in different ways depending on which ingredients they contain.

Appetite suppressants such as sibutramine (Meridia), diethylpropion (Tenuate) and phentermine (Adipex-P, Fastin, Anoxine-AM, etc.) affect the appetite-regulating region of the brain called the hypothalamus. They work by blocking the re-uptake of the chemicals serotonin and norepinephrine, which create that feeling of satiety you get after eating a big meal. With more of these chemicals circulating in your brain, you feel full, so you eat less.

Prescription fat blockers, such as orlistat (Xenical), inhibit the action of an enzyme called lipase. When we eat foods with fat in them, lipase normally breaks down the fat when it gets to the intestinal tract. With a lipase inhibitor in effect, a percentage of the fat ingested is removed from the body through bowel movements instead of being broken down and absorbed.

Many prescription drugs are used as diet aids even though that was not their intended purpose. Some antidepressants are used "off-label" as diet aids because they have been shown in studies to help patients lose weight and keep it off for several months. Researchers are also studying certain drugs normally used to treat epilepsy (topiramate and zonisamide) and diabetes (metformin) for their weight-loss potential.

Because diet pills are such a lucrative business, pharmaceutical companies are pushing to introduce more products to the market. More than 100 new weight-loss medications are either in development or in clinical trials as of January 2005. One of the most promising is rimonabant (Acomplia), which acts on a protein in the brain called the endocannabinoids. Endocannabinoids are similar in structure to the active ingredient in cannabis (marijuana) and may be responsible for appetite control. Rimonabant blocks the endocannabinoids from reaching their receptors in the brain. (See Study: Marijuana-like substances control appetite to learn more.) By blocking this protein, the drug reduces food cravings. In clinical trials, one-third of obese people who were given Acomplia lost more than 10 percent of their body weight and were able to keep it off for up to two years. The drug also has a few welcome side effects: It increases HDLs ("good cholesterol") while lowering triglycerides (a form of fat that is carried in the bloodstream). Acomplia may even help smokers kick the habit.

Other promising drugs affect appetite-related hormones. One blocks ghrelin, which the stomach sends to the brain to increase appetite. Another mimics a hormone called PYY, which tells the body it's full.

Who Should Use Prescription Diet Pills?

Prescription diet pills aren't for the person who wants to shed a few pounds to fit into a holiday party dress or tuxedo. Only people who are "obese" (those who are 30 percent over their ideal weight, or have a Body Mass Index (BMI) of 30 or more -- see National Heart, Lung and Blood Institute: Healthy Weight to calculate your BMI) or who have a history of high blood pressure or diabetes are good candidates for prescription diet medications, according to the National Institutes of Health (NIH).

Even though approximately 16 percent of American children are overweight, most diet pills are not recommended for use by children under the age of 16. The exception is orlistat, which can safely be used by teens 12 and older.

If you watch late-night TV, you know that it seems like every other ad is touting the miraculous claims of the latest over-the-counter diet pill. Read on to find out about OTC diet pills.