Medically reviewed by faculty at the UC Berkeley School of Public Health

View as List What is in Your Diet Pills?

Weight loss supplements contain a mind-numbing list of substances touted to burn fat, boost metabolism, suppress appetite, control sugar cravings and increase energy. Some claim to help you lose weight by elevating your mood or helping you overcome a genetic predisposition to obesity. A few ingredients have been tested in controlled studies, with inconsistent results. But most have no convincing scientific support behind them or have not been tested at all. Get the rundown on nine common diet supplement ingredients here.
1

Caffeine

Diet pills often contain substances, such as green tea, guarana and yerba mate that claim to have “natural” stimulant properties. But what you’re really getting is caffeine or caffeine-related substances. Caffeine may temporarily boost calorie burning, but this does not mean you will lose much weight. The effects are small and temporary and caffeine tolerance tends to develop with regular use. Moreover, if a diet pill does boost metabolism, then it can also boost heart rate and blood pressure and have other potentially undesirable effects.
2

Capsaicin

Some research suggests that capsaicin and related substances in chili peppers can briefly speed metabolism, increase fat burning and suppress hunger. However, the studies tend to be small, short and poorly designed and have had inconsistent results. There’s no evidence that such supplements lead to significant and lasting weight loss.
3

Conjugated linoleic acid

Studies on conjugated linoleic acid (CLA), a fatty acid in dairy and meat, have yielded mixed results. A 2010 report from Health Canada (which acts like our FDA) said there is insufficient evidence that CLA supplements help with weight loss. Potential adverse effects include increased inflammation, decreased HDL (“good”) cholesterol and reduced insulin sensitivity. CLA has been shown to cause liver damage in animal studies. It's unclear which formulations (there are many) are riskier.
4

Chitosan

This fiber-like substance, derived from crustacean shells, is said to block fat absorption in the intestines. Although there is some evidence that chitosan may help in the short term, studies have had mixed results and most are poor quality. Better-quality trials have found no clinically meaningful effect on weight. Plus, there are potential risks. If Chitosan does block some fat, it may also reduce absorption of some fat-soluble vitamins (A,D,E,K), as well as calcium, iron and magnesium. People with shellfish allergies should not take chitosan.
5

Diuretics

Some ingredients, such as green coffee, cherry stalk and wild pansy, act as diuretics, which increase urination. This may lead to small initial weight loss, but it’s only water loss, and the weight comes back as soon as you rehydrate. Diuretics do not cause fat loss.
6

Food Extracts

Products that contain extracts of foods such as broccoli, spinach, blueberries, garlic, sprouts, tomatoes and soybean have been touted by some companies to help you lose “10 pounds in 10 days.” These ingredients have no known weight-loss benefits and are typically present in very small amounts. Papaya and pineapple are sometimes included, too, presumably for their digestive enzymes, though these have nothing to do with weight loss either.
7

Fiber

Fiber does help fill you up, so you are more likely to eat less. But the amounts in diet supplements tend to be quite small. If you want to supplement your diet with more bulk-forming fiber, generic psyllium is a good (and cheaper) choice.
8

Green Tea Extract

Besides caffeine, green tea contains polyphenols called catechins, which may promote calorie and fat burning. A review found that catechins decreased weight and helped keep it off. But not all studies confirm the benefits. The effects of green tea extracts may vary from person to person, and if there is weight loss, it’s likely to be small at best.
9

Herbal laxatives

Cascara, senna, rhubarb root, buckthorn and aloe (often in “dieter’s teas”) are promoted for quick weight loss, under the mistaken notion that they expel undigested food and thus calories. In fact, these ingredients simply stimulate your bowels and have little or no effect on absorption of food. Chronic use may lead to dependency, decreased bowel function, dehydration and electrolyte imbalance; some deaths have been reported from laxative abuse.