Though we often take them to stay healthy, looking for protection from heart attacks, cancer, bone fractures, and more, most supplements have not been proved to improve or prevent those conditions. Here’s a quick review of seven of the top-selling supplements, including how they’re used, known benefits, and potential risks.
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Used for: Promoting general health; preventing chronic conditions such as cancer and cardiovascular disease.
The evidence says: Multis can help prevent nutrient deficiencies in specific groups, such as pregnant women. But clinical trials repeatedly have failed to prove that the pills benefit healthy people.
Possible risks: It’s possible to overdose on certain nutrients such as vitamin A and calcium, which many multis contain. So keep track of how much you get in total from your diet, including fortified foods such as cereal.
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Used for: Ensuring healthy metabolism; boosting energy.
The evidence says: Food provides plenty of the B vitamins—including B1 (thiamine), B2 (riboflavin), B3 (niacin), B6, B12, and folic acid—for most people. Exceptions include vegetarians, people who have trouble absorbing B12, and women who are or may get pregnant (they need extra folic acid from a prenatal vitamin or other supplement).
Possible risks: High doses of folic acid can mask B12 deficiency.
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Used for: Promoting healthy blood clotting and protein synthesis; preventing cancer.
The evidence says: Leafy greens and other vegetables provide most people with all of the vitamin K they need. Even if you don’t eat veggies, the “good” bacteria in your intestines also make the vitamin. There’s no clear evidence that supplemental doses prevent cancer.
Possible risks: Too much vitamin K can make the blood-thinning drug warfarin (Coumadin and generic) less effective.
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Used for: Cardiovascular protection.
The evidence says: Eating two or more servings per week of oily fish is linked with a lower risk of heart attacks and strokes. But the latest evidence suggests that fish-oil pills don’t offer the same protection, even for people at risk for heart disease.
Possible risks: Fish oil can thin the blood, so talk to your doctor before taking it, especially if you take a blood-thinning drug.
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Used for: Building healthy bone; preventing osteoporosis and fractures.
The evidence says: The U.S. Preventive Services Task Force says there’s not enough evidence to show that calcium supplements prevent fractures in either premenopausal or postmenopausal women who take more than 1,000 milligrams per day.
Possible risks: Some evidence has linked a high intake of calcium to a greater risk of heart disease, but the findings have been inconsistent.
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Used for: Preventing or treating colds; protecting against cancer and heart disease.
The evidence says: People who eat foods high in vitamin C appear to have lower rates of heart disease and some cancers. But studies of supplements have mostly failed to show any big benefits. There is evidence that 200 milligrams or more per day may improve cold symptoms in seniors and smokers.
Possible risks: High doses can boost iron absorption, so avoid supplements if you have hemochromatosis, in which iron builds up in the body.
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Used for: Relieving arthritis pain.
The evidence says: The supplements helped some people with osteoarthritis in a major 2006 clinical trial, but when results were averaged over two years, they weren’t significantly better than a placebo at slowing the loss of cartilage. More recently, a 2014 study showed that glucosamine didn’t relieve knee pain or cartilage loss in people with osteoarthritis.
Possible risks: Some people with shellfish allergies might need to avoid glucosamine.
This article also appeared in the June 2014 issue of Consumer Reports on Health.
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