(FORTUNE Magazine) – It's 7 A.M., breakfast time. I'll start with Vitamin E and folic acid for my heart, add ginkgo biloba so that I don't forget which pills to take later, and wash them down with St. John's wort so that I don't get depressed by all the money I'm spending on supplements.

Don't laugh. Last year Americans shelled out more than $2 billion on multivitamins and $12 billion on a long list of other vitamins and dietary supplements. An estimated 60% of adults pop supplements, and the percentage is growing rapidly. But while many doctors and nutritionists are enthusiastic, others find supplements worthless. Who's right?

First, let's define what we're talking about. In addition to vitamins and minerals, dietary supplements include amino acids, herbs, nonnutritive substances (such as chitosan, made from the exoskeletons of shellfish), and even hormones like DHEA, which couldn't be marketed prior to the 1994 passage of the Dietary Supplement Health and Education Act--legislation that says a dietary supplement does not have to prove that it works, or even that it's safe. In fact, a problem has to occur before the Food and Drug Administration can step in. This is caveat emptor in its truest form.

On the other hand, studies have shown that some supplements are beneficial. Calcium and vitamin D help prevent osteoporosis and aren't a bad idea for women. Most people swear by vitamin C as a way of shortening colds by a day or so. Glucosamine and chondroitin sulfate appear to relieve pain and improve mobility in people who have osteoarthritis. St. John's wort seems to help with depression. Garlic lowers cholesterol, thins the blood, and reputedly keeps vampires away.

Kids chewed $300 million worth of vitamins in 1999, but the elderly probably consumed more. An estimated 15% of people over 65 have early vitamin B12 deficiency. Ginkgo biloba may improve memory slightly in people with dementia; it's being marketed to the general population, however, and I've seen no evidence that it improves memory in "normal" people. Vitamin E may also improve memory slightly in people with dementia, and a simple multivitamin and vitamin E may improve immune function.

Folic acid, proven to decrease the risk of neural tube defects during pregnancy, may protect against heart disease by lowering homocysteine levels. One study showed that selenium prevented different types of cancers, but these findings need to be replicated by other researchers. And then there's Echinacea, which may boost immune function in the short term and help fight colds--but might also suppress immune function if it's taken continuously.

While the benefits of supplements are uncertain and hard to prove, some have clear-cut risks. Ephedra, a stimulant added to many weight-loss products, can raise blood pressure and cause abnormal heart rhythms; deaths have been reported. One of my patients had abnormal liver tests; the cause was chaparral, an herbal supplement that supposedly prevents cancer. Vitamin E, garlic, ginseng, ginkgo biloba, and ginger can increase bleeding, particularly when combined with other blood thinners. Despite a lot of hoopla, the most reliable study to date suggests vitamin E doesn't do much to prevent heart disease. And it now appears that St. John's wort can decrease the effectiveness of other drugs, including oral contraceptives and the heart medication digoxin.

Given all of the above, a healthy diet is your best bet. We'll never be able to pack into a pill all the nutrients Mother Nature puts into foods. Second, do your homework. Before adding a supplement, research its benefits and risks--realizing that we don't have all the answers, because many supplements haven't been studied enough. Third, tell your doctor. Some supplements interfere with medications; others cause problems during surgery. If the supplement has "USP" on the label, it's been okayed by the U.S. Pharmacopoeia, a nonprofit organization that tests for strength, purity, and the like. Also, sticking with a name brand is wise: A large company has more to lose.

A good start would be a multivitamin with minerals that contains 100% of the Recommended Dietary Allowance, especially if you're over 65 or trying to get pregnant. But even with a complete multivitamin, about the most that can be said is that it might help and it probably won't hurt.

FOR MORE INFORMATION ON THIS TOPIC: Go to mayoclinic.com. Mayo Clinic offers Executive Health programs at Mayo Clinics in Jacksonville, Fla.; Rochester, Minn.; and Scottsdale, Ariz.