Many types of diet pills claim to treat obesity and help people to lose weight. Some have been approved by the Food and Drug Administration (FDA) to treat obesity and are available only by prescription. Others are available over the counter and may contain chemicals or herbal ingredients that might or might not be effective.

Most diet pills are classified as appetite suppressants. They work by altering the levels of two chemicals in the brain called serotonin and catecholamine. These chemicals are associated with numerous aspects of mood and behavior, including appetite. By increasing the levels of serotonin and catecholamine, the body is tricked into thinking that it is not hungry. However, regular exercise and a healthy diet must accompany any type of weight-loss treatment plan in order to maintain weight loss.

Appetite suppressants that have been approved by the FDA to treat obesity include:

• Diethylpropion (Tenuate, Tenuate dospan)
• Mazindol (Sanorex, Maznor)
• Phendimetrazine (Brontril, Plegine, Prelu-2, X-Trozine)
• Phentermine (Adipex-P, Fastin, Ionamin, Oby-trim, Pro-Fast, Zantryl)
• Sibutramine (Meridia)

Another FDA-approved medication for obesity is orlistat (Xenical). This type of diet pill is a lipase inhibitor, which works differently than appetite suppressants. Rather than reducing hunger, lipase inhibitors block the action of an enzyme called lipase. Lipase is responsible for breaking down fat in the intestines so it can be easily absorbed by the body. If this action is blocked, the body cannot efficiently absorb the fat, reducing the amount of calories derived from food. The fat is instead excreted in the feces. Orlistat is available by prescription or in a lower dose without a prescription (Alli).

In addition to the diet pills that have been tested and approved by the FDA, there are a wide variety of over-the-counter drugs that claim to treat obesity. Some over-the-counter diet pills may be unsafe, especially for people who have additional medical conditions (e.g., diabetes, high blood pressure) or who are taking other medications. Some of the most common over-the-counter diet pills and ingredients include:

• Ephedrine. Ephedrine (or Ephedra) has been sold as both an energy enhancer and a “fat burner.” Studies have shown that ephedrine does indeed increase the body’s ability to burn fat and may result in short-term weight loss. However, recent studies have shown that ephedrine has toxic effects with short-term and long-term use. Controversy mounted in 2003 after the sudden death of a professional baseball player who was taking the drug. In 2004, the FDA banned the sale of products containing ephedrine, although it may be legally sold in tea. The herb ma huang is a source of ephedrine.
  
  
• Chromium picolinate. A number of people take this substance to treat obesity and diabetes, though it has not been approved by the FDA for either purpose. The FDA does recommend that people consume 120 micrograms of chromium daily, a tiny amount, because scientists found that this mineral promotes the action of insulin in transporting glucose (blood sugar) to fuel the body’s cells. Some studies have suggested that chromium may help control glucose.
  
  The combination of chromium with picolinic acid promotes the body’s absorption of chromium. Research has produced mixed results on whether chromium picolinate can promote weight loss. It has been linked to kidney problems, gene mutations and greater risk factors for developing cancer.
  
  
• Carnitine. Carnitine is a nutrient found mostly in red meat and dairy products. It helps the body convert fatty acids to energy. Supplements such as L-cartinine (levocarnitine) have been touted as a weight-loss aid, but scientific research has not proven these claims. However, scientists are studying whether L-carnitine might benefit some patients, including people receiving hemodialysis for kidney disease and people with heart disease, hyperthyroidism, dementia, male infertility or some other conditions.
  
  
• Hoodia. This African desert plant has been touted as an appetite suppressant. However, even if it is effective, some products claiming to contain hoodia have been found to contain very little or none of the herb.
  
  
• Bitter orange. Also known as citrus aurantium, bitter orange is an herbal supplement derived from a type of orange. It contains chemicals that are similar to the banned substance ephedrine, and use may result in side effects including high blood pressure and arrhythmia (irregular heartbeat).
  
  
• Pyruvate. The body’s metabolism of glucose produces this substance. Limited research has suggested that large doses might help promote modest weight loss, but side effects may include diarrhea and upset stomach.
  
  
• Chitosan. Chitosan is a chemical compound that is claimed by some to block the absorption of fat by the body. It is primarily used in agriculture as a plant growth enhancer. Although chitosan is generally safe to use, studies have shown that it is not effective as a diet pill and may result in the malabsorption of certain vitamins and minerals.
  
  
• Green tea. This supplement, taken as a pill or as a tea, has been shown to slightly increase the body’s ability to burn fat and may result in modest, short-term weight loss. Green tea may provide other health benefits because it contains antioxidants. However, green tea may negatively interact with some medications.
  
  
• Salicin and white willow. These ingredients are related to aspirin, which may have negative effects in some patients, such as those with diabetic nephropathy or other kidney disease.

Diet pills that have been withdrawn because of health concerns include dexfenfluramine (Redux), phenylpropanolamine (PPA), and the combination of fenfluramine and phentermine (fen-phen). The FDA has also banned supplements containing certain ingredients, including Aristolochia plant species, which are toxic to the kidneys, and guar gum, which in some cases has blocked the digestive tract.

Many over-the-counter diet pills contain stimulants (e.g., caffeine, ginseng, guarana) that are designed to speed up the body’s metabolism and aid weight loss. However, these products often contain large doses of stimulant that may result in adverse side effects, such as irritability, sleeplessness, high blood pressure and cardiovascular problems. Additionally, some research has shown that caffeine may be especially dangerous for people with diabetes because it can aggravate hypoglycemia and has been linked to insulin resistance. However, other studies have found that caffeine may help prevent diabetes or control glucose.