Diet pills are drugs taken to control weight. They usually work as appetite suppressants – decreasing hunger or making the body feel full more quickly than usual after eating. Some diet pills may increase the body’s metabolism to burn more fat, a process known as thermogenesis.

Weight control is an important aspect of preventing and managing numerous health conditions, such as diabetes, high blood pressure and heart conditions. However, diet pills should be used only under the strict supervision of a physician and in conjunction with a healthy diet and exercise plan.

Most diet pills approved by the Food and Drug Administration (FDA) are effective only for short–term use, such as a few weeks. Weight loss of 10 to 15 percent is considered a good response to the drugs. Patients may be disappointed if they believe that these drugs can remove unwanted fat and keep it off forever. The amount of weight lost while taking these drugs is usually less than people hope for, although some patients have lost more than 20 pounds.

Studies have shown that most people experience modest weight loss (sometimes less than 5 pounds) for the first six months after beginning treatment to lose weight. After the first six months, many patients stop losing weight. Unless patients have developed a healthy pattern of eating and exercise, they can expect to regain weight lost while taking diet pills. Even when long-term use of diet pills is approved by a physician, it must be accompanied by a healthy diet and exercise to reap the full benefits of such treatment.

The FDA has approved only two diet pills for long-term treatment of obesity: sibutramine (Meridia) and orlistat (Xenical). Sibutramine is an appetite suppressant. Orlistat is a lipase inhibitor and works in a different way than sibutramine and other appetite suppressants. Lipase inhibitors reduce the amount of fat that can be broken down in the body, making it harder for the body to absorb the fat. In 2007, a lower-dose version of orlistat (Alli) became the only over-the-counter diet pill approved by the FDA.

In addition to FDA-approved diet pills, there are a wide variety of over-the-counter and herbal supplements that are marketed as a treatment for obesity. Some of these drugs even claim to promote weight loss without the patient having to make any lifestyle changes.

Although some of these products may result in short-term weight loss, there is no evidence that any non-FDA-approved drugs can produce significant, long-term weight reduction. People generally regain weight they lost when they stop using the pills, and this “yo-yo effect” has been shown to promote weight gain over time. Some over-the-counter diet pills have also been associated with a number of serious health risks and may be especially dangerous for people with certain conditions, including diabetes, thyroid disease or cardiovascular problems.

Many diet pills, including those that are approved by the FDA, may have side effects that are dangerous for people with diabetes or other disorders. Some diet pills (e.g., ephedra, fenfluramine) have been taken off the market after studies showed that they caused life-threatening health problems. It is important that patients consult their physician before taking any type of medication or supplements.

The FDA warns against using hormonal thyroid medications (a treatment for hypothyroidism) in an attempt to treat obesity in people with normal thyroid function. High doses of these medications may cause serious or even life-threatening toxicity, especially if combined with amphetamines.

The FDA has also warned against purported weight-loss supplements that actually contain prescription medications or other potent substances, such as:

• Strong thyroid hormones that could cause heart attack, stroke or other serious conditions
  
  
• A potentially deadly combination of stimulants, hormones, tranquilizers and laxatives
  
  
• Antidepressants or other psychiatric medications

Some medications, including diuretics, laxatives and emetics (vomit inducers), and illegal drugs are also sometimes abused in an attempt to lose weight. Such misuse can have dangerous consequences.

A new class of injected antidiabetic agents known as incretin mimetics has promoted weight loss in many patients and sparked interest from nondiabetics. However, it is approved only to treat type 2 diabetes and is not to be used as a weight-loss aid in nondiabetics. Metformin, which is in the biguanide class of antidiabetic agents, has helped some patients lose weight by promoting a feeling of fullness after meals. It is sometimes prescribed off-label for prediabetes, insulin resistance or polycystic ovarian syndrome, conditions that typically involve excess weight.