Drugstores, nutrition shops and Web sites offer a plethora of supplements and combination products that claim to help burn fat, block fat absorption, modulate carbohydrate metabolism, boost energy expenditure, suppress appetite, flush out excess water weight and control cravings -- all good outcomes to an eager dieter.

So what's the harm in grabbing a bottle?

"There's a tremendous appeal for a magic pill to help lose weight -- manufacturers of weight-loss pills appeal to that, I think," said Dr. Robert Saper, director of integrative medicine in the Department of Family Medicine at Boston University Medical Center.

But in a review of the scientific evidence, published in the Nov. 1, 2004, issue of American Family Physician , Saper suggested there's no miracle pill on the market. In fact, some of the supplements out there may be dangerous or might interact with prescription medications that people are taking, he said.

"There's very few of them that show benefit for what people are taking them for," agreed Dr. Gary A. Green, a clinical professor in the Division of Sports Medicine at the University of California, Los Angeles.

Steven Dentali, vice president of scientific and technical affairs at the American Herbal Products Association, offers a somewhat more upbeat assessment: "I would say there is some preliminary evidence that some of these ingredients can be useful in combination with diet and exercise."

Dietary supplements have quite a following among many Americans. Overall, 7 percent of adults use over-the-counter weight-loss supplements, according to a five-state survey reported in 2001 in the Journal of the American Medical Association . The use of nonprescription weight-loss products is particularly common among young obese women -- 28.4 percent reported taking them.

One recent success story is that of former Playboy "Playmate of the Year" Anna Nicole Smith. The 37-year-old actress and model dropped 69 pounds in eight months using a formulation of TRIMSPA, a popular brand-name product, the supplement maker's Web site claims.

But do you know what you're getting when you buy brand-name dietary supplements, including TRIMSPA, Xenedrine, and Hydroxycut, or individual herbal products?

The Natural Medicines Comprehensive Database, a source for evidence-based information on natural brand-name products and ingredients, lists more than 50 individual supplements and 125 proprietary products, according to Saper, who examined individual ingredients found in several commercial products.

Chromium, for example, is a popular weight-loss supplement found in many products, but its efficacy and long-term safety are uncertain, he said. Guar gum, derived from the Indian cluster bean, appears to be ineffective for weight loss. Chitosan, derived from shellfish, is another one to avoid, Saper said. "I discount chitosan because, although it's safe, its role as a 'fat blocker' is not well-substantiated," he explained.

Glucomannan, a plant fiber, may be helpful for modest weight loss, he said, but the data are insufficient to support a definitive conclusion.

Overall, the amount of well-designed research to substantiate the effectiveness of many individual herbs for weight loss is sparse, Saper said. Even greater questions exist as to the safety and effectiveness of combination products and the potential for harmful interactions among different ingredients, he noted.

Dietary aids containing the herbal supplement ephedra were extremely popular until the U.S. Food and Drug Administration banned their sale in April 2004 due to reports of deaths. Ephedrine, the active ingredient, is a stimulant known to increase metabolic rate, heat production and the risk of heatstroke.

Dangerous supplements still remain available on store shelves and Web sites, Consumers Union warns. The public advocacy group's Consumer Reports magazine last May published a list of the "dirty dozen" -- 12 supplement ingredients that have been linked either to serious adverse effects or to a strong theoretical risk. The list included bitter orange, a common dietary supplement that contains a compound called synerphrine, which carries risks similar to ephedrine, Saper said.

Critics of the dietary supplement industry want Congress to put some teeth into federal regulations. Under the Dietary Supplement Health and Education Act of 1994, manufacturers of dietary supplements are not required to provide evidence of safety or effectiveness. The burden rests on the FDA to prove that a product is either unsafe or ineffective.

"As long as they don't claim to treat any specific disease or condition they can basically make any claims that they want," Green said.

For its part, the herbal products association has expressed support for a mandatory adverse-events reporting system for dietary supplements.

Until there's better data, motivated dieters would be better off talking to their physicians about proven weight-loss strategies, such as diet and exercise, according to Saper.

But Dentali believes herbal products can play a role in a sensible weight-loss strategy. Switching from soda to green tea, for example, may provide some health benefits, "and you're not sucking down those sodas."

SOURCES: Robert Saper, M.D., director, integrative medicine, Department of Family Medicine, Boston University Medical Center; Gary A. Green, M.D., clinical professor, Division of Sports Medicine, University of California, Los Angeles, and Pacific Palisades Medical Group, Pacific Palisades, Calif.; Steven Dentali, Ph.D., vice president, scientific and technical affairs, American Herbal Products Association, Silver Spring, Md.; Nov. 1, 2004, American Family Physician ; Aug. 22, 2001, Journal of the American Medical Association