People turn to nutritional supplements for some perfectly good reasons, including a need to compensate for an inadequate diet, a desire for a "natural" cure, or hope when medical science has no answer. And sometimes supplements may be of some help. For example, research suggests that probiotics may relieve intestinal problems and possibly even the common cold, ginger may ease nausea, and coenzyme Q10 may help prevent migraines and lower blood pressure. But the public is often surprised to learn that supplements lack many of the safeguards afforded to prescription or over-the-counter drugs, and that even potentially helpful supplements can pose risks. Here's how to find the supplements that may help you--and how to protect yourself from those that almost certainly won't.

A few dietary supplements, such as the herbs kava and yohimbe or the hormone DHEA, are potentially so dangerous that Consumer Reports' consultants say they shouldn't be sold at all. Others can be risky for certain people or in large doses or when combined with certain drugs. And even those that are probably safe may not help much. But unlike medications, the Food and Drug Administration (FDA) doesn't require that dietary supplements be proved safe and effective before they go on the market, and it barely oversees them once they get there.

That means you have to learn all you can about the supplements you're thinking of taking. Unfortunately, much of what's on the Internet--and what the store clerk tells you for a quick sale--may be inaccurate. Here are some better sources:

• Consumer Reports Health uses information from the independent Natural Medicines Comprehensive Database to provide safety concerns and effectiveness ratings for thousands of dietary supplements. Available by subscription.
• The National Institutes of Health Office of Dietary Supplements.
• The National Center for Complementary and Alternative Medicine.
• The National Library of Medicine's Medline Plus.

Labels that sound too good to be true--"boundless energy," "quick weight loss," "cancer cure"--almost certainly are. But even those that seem more plausible, such as "promotes prostate and urine-flow functions," or "supports the immune system," are of dubious value. Manufacturers can make those claims without submitting any proof, provided that the label describes how the product affects the body's "structure or function" rather than how it prevents or treats disease, and as long as the label states that the FDA did not review the claim. And even when manufacturers have supporting evidence, it often comes from preliminary studies, not the rigorous trials required for drugs. So choose supplements based on your research, not on label claims.

It's possible to overdose even on vitamins and minerals, which are by far the safest and best studied supplements available. High doses of calcium, for example, can impair the absorption of other minerals, cause kidney stones, and possibly increase prostate cancer risk in men. And just twice the Daily Value of vitamin A (from retinol) can increase the risk of birth defects and liver damage. Recommended allowances and tolerable upper limits for vitamins and minerals can be found on the Institute of Medicine's Web site. For herbal and other supplements, it's often difficult to know what the optimal dose should be--but avoid any product that boasts of "megadoses" or extra-high amounts.

Supplements can interfere with some medications and make side effects of others more likely. St. John's wort, for instance, may undermine drugs used to treat HIV/AIDS, asthma, high-cholesterol levels, and hypertension. Garlic, ginger, and ginseng all increase the risk of bleeding from blood-thinning drugs such as aspirin, clopidogrel (Plavix) or warfarin (Coumadin). Supplements can also pose risks to people with underlying health problems. Echinacea, for example, may worsen rheumatoid arthritis, glucosamine may complicate diabetes, and black cohosh may be a growth factor for uterine fibroids and breast cancer.

So if you have a chronic disease or take medication and are thinking about trying a supplement, talk with your doctor and pharmacist first. And check other sources too, since your doctor may not know all the answers. On Consumer Reports Health, for example, you can search for interactions between specific supplements and drugs and see the conditions that each supplement may worsen.

The FDA requires supplements to bear an ingredients label--but doesn't verify that information or monitor manufacturing processes. And tests by Consumer Reports and others have found that bottles don't always contain what their labels say they do or in the dosage they purport to have. Some have even been contaminated with heavy metals, pesticides, bacteria, or prescription drugs.

To avoid those problems, look for products with "USP Verified" on their label. The manufacturers of those supplements have voluntarily asked the U.S. Pharmacopeia, a nonprofit organization that sets standards for pharmaceuticals in the U.S., to test their products for identity, purity and potency, and to review their manufacturing processes as well as their package inserts. For a complete listing of products verified by USP, visit www.usp.org/USPVerified/dietarySupplements/supplements.

The Natural Medicine Ratings on Consumer Reports Health identify those products, which you can readily find in many places where supplements are sold. But if you can't locate one, it's generally best to stick with a well-known brand. Our tests found that the contents of those products usually have the amount stated on the bottle's label and dissolve adequately, even if they haven't yet undergone USP testing, while many "dollar-store" brands don't.