Diet Pills: What You Need to Know

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.

Some over-the-counter pills (such as Xenedrine, Dexatrim and Zantrex-3) once used a powerful combination of the stimulant ephedra and caffeine to help users shed pounds. They reportedly worked in combination by increasing energy expenditure -- or thermogenesis (it is thought that caffeine increases metabolism by increasing the breakdown of fatty acids) -- and reducing appetite (ephedrine acts on the satiety center in the hypothalamus). But since late 2003, when the FDA banned the sale of ephedra-containing drugs due to evidence of increased risk of heart attack, these formulations have replaced ephedra with a mixture of vitamins (such as thiamin, riboflavin, vitamin B-6, folic acid), an ephedra-like substance called synephrine that is found in certain citrus fruits, and caffeine. The combination reportedly increases energy while boosting metabolism.

There are many herbal weight-loss supplements on the market today, and they utilize different mechanisms to reportedly aid in dieting. Here are some of the more common ingredients and their reported weight-loss mechanisms:

So as you can see, there are a lot of diet pills out there, all claiming to help you lose weight in one way or another. But do they really work? And what kinds of side effects can you expect?

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Many over-the-counter diet pill manufacturers say their product will help you see miraculous weight loss -- like losing up to 30 pounds in 30 days -- without diet or exercise. Their claims sound too good to be true, and most of them are.

A few pills, especially the newer prescription varieties (such as Meridia and Xenical), have been shown in clinical studies to help dieters shed a few pounds. But the majority of the ads you see on the Internet and TV are for products that are unregulated, untested and unproven.

Even the most effective diet pills are only meant to be taken for a short period of time -- usually six months or less. During that time, doctor-prescribed weight-loss drugs can trim anywhere from 5 to 22 pounds, or up to 10 percent of your body weight. But after six months, your body develops a tolerance to these drugs' effects, and weight loss plateaus. After that, if you don't also follow a healthy eating and exercise plan, the weight will come right back.

Side Effects of Diet-pill Use

Different diet pills contain different ingredients, so their side effects vary:

As fat blockers like orlistat (Xenical) remove excess fats via the intestines, they may cause uncomfortable cramping, gas and diarrhea. Because these drugs also reduce the body's absorption of essential vitamins and nutrients, people who take Xenical are advised to take a daily multivitamin supplement.

Sibutramine (Meridia) and other similar appetite suppressants stimulate the sympathetic nervous system, which can raise blood pressure and heart rate. This increases the risk of heart attack and cardiac arrest, especially among people who already suffer from high blood pressure, irregular heartbeat or heart disease. In fact, between February 1998 and March 2003, the FDA received reports of 49 deaths related to sibutramine. Other, more minor side effects include constipation, headache, dry mouth and insomnia (because the chemicals in these drugs also influence sleep patterns).

Herbal diet pills, even though they're "all natural," can have potentially dangerous side effects depending upon their ingredients. "Herbal" doesn't necessarily mean "safe." Also, because they are considered part of the food industry and are therefore regulated differently by the FDA, there is no guarantee that they can live up to their manufacturers' claims.

To find out more about diet pills and related topics, check out the links on the next page.