More than half of American adults take nutritional supplements, but they are not without risk. The Food and Drug Administration has received more than 6,000 reports of serious adverse events in the past five years.

A Consumer Reports investigation has found many surprising dangers in vitamins and supplements, which are largely unregulated products. Moreover, there’s increasing evidence that there may be few, if any, benefits.

For example, a study published in June showed that calcium supplements increased the risk of heart attack by 86 percent compared with the group who didn’t get them. On the other hand, that same study showed eating calcium-rich foods can protect your heart.

A recent study of antioxidant supplements showed that high doses of some “may increase cancer risk” and not reduce it.

Even more troubling, some supplements have turned out to contain prescription drugs, such as Viagra and Cialis, that were not on the label. The FDA says supplements spiked with prescription drugs are “the largest threat” to consumer safety. There have been more than 400 recalls of such products since 2008.

Even with uncontaminated vitamins and minerals, the labels don’t tell the whole story because generally, the FDA does not require manufacturers to include warning labels.

When Consumer Reports checked out the labels on more than 200 bottles of supplements, it found just one in three listed possible adverse reactions.

In some cases, the potential risks of supplements outweigh their benefits. Consumer Reports says that if you’re generally healthy, you can skip them. Eating a diet that includes fruits, vegetables, grains, dairy, and protein is the best way to go.

However, for those on a restricted diet or with certain medical conditions, there are some supplements you may need to take. Of course, check with your doctor first.

Government experts say reports of problems related to supplements are quite likely to be underreported. If you experience a problem, you can report it to the Food and Drug Administration at www.fda.gov/Safety/MedWatch or 800-332-1088.

Consumer Reports' complete investigation is available online.