To answer this question it must be understood by the reader that the term "diet pills" is a catch-all for any pill given for the purpose to help with weight loss. They include medications which act in very different ways. I have tried to summarize these below. 1. Appetite Suppressants - These are available by prescription as sibutramine (Meridia), diethylpropion (Tenuate) and phentermine (Adipex-P, Fastin, Anoxine-AM, etc.) and 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. 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. Other, more minor side effects include constipation, headache, dry mouth and insomnia (because the chemicals in these drugs also influence sleep patterns). 2. Fat Blockers - Also available by prescription include orlistat (Xenical), and 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. 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. 3. "Off Label" - 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. Another popular diet pill ingredient in the '90s was a powerful amphetamine-like stimulant called ephedra (or the Chinese herb ma huang), which, in combination with caffeine, triggered measurable weight loss. The problem was, because it sped the heart rate, ephedra also dramatically increased the risk of heart attack and stroke. Ephedra is related to epinephrine (adrenaline), which, during times of stress, constricts blood vessels, elevates heart rate and gets the body ready to fight or flee. At least 155 people died from taking medications containing ephedra. 4. Over The Counter - 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. 5. Herbal - 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: - Herbal ephedra, caffeine, guarana and country mallow - increase metabolism. - Guar gum, glucomannan and psyllium - Cause a "full" feeling - Hydrocitric acid, green tea, conjugated linoleic acid and pyruvate - Slow fat production - Chitosan (or chitin - substance found in the exoskeleton of shrimp and other shellfish) - Block fat from being absorbed 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. Because all of the various "diet pills" can have serious side effects, they should only be taken under the monitoring and supervision of a physician.