Herbal, vitamin and mineral pill products in a file photo. Credit: Reuters/File

In an undercover probe, investigators at the Government Accountability Office also found that labels for some supplements claim to prevent or cure ailments like diabetes or heart disease -- a clear violation of U.S. law.

GAO staff targeted supplements most popular with older consumers and posed as elderly buyers in stores or over the telephone.

The most egregious practices included suspect marketing claims that a dietary supplement prevented or cured extremely serious diseases, such as cancer and cardiovascular disease, the GAO said in a report released on Wednesday at a Senate hearing.

For example, one shopper at a supplement specialty store was told that a garlic supplement could be taken instead of prescribed blood pressure drugs. Another staffer posing as a forgetful, elderly consumer was told by a salesperson that he could take aspirin and ginkgo biloba together with no harm. The Food and Drug Administration has said that combination can cause internal bleeding.

The GAO, which conducts investigations for Congress, also said it found trace amounts of potentially harmful contaminants such as lead and arsenic, but at levels that do not exceed federal guidelines.

Findings of pesticides, however, did exceed the FDA's advisory levels, the GAO said, and 16 of 40 supplements tested would violate the FDA's tolerance.

Manufacturers told the GAO that they were concerned about the contamination findings but that the levels identified were too low to raise any issues during their own testing.

Given the expected generally small consumption of the supplements, we do not believe these levels represent a significant risk to health, FDA Deputy Commissioner Joshua Sharfstein said in remarks prepared for the Senate Committee on Aging.

Unlike prescription drugs, the FDA does not approve dietary supplements before they can be sold. The Federal Trade Commission regulates the marketing of herbal supplements, which are not allowed to claim that they treat medical conditions.

But calls for stronger oversight have been growing.

Experts at the Institute of Medicine have said the FDA needs to use the same strict standards to regulate supplements as it uses for medications, and earlier this week the GAO said the FDA should ask Congress for more power to regulate supplements.

Some lawmakers, including Republican Senator John McCain of Arizona, have proposed legislation to beef up the agency's oversight of supplements.

Sharfstein said the FDA is obliged to look for problems only after a product is sold by reviewing medical literature and analyzing complaints. The agency has taken action to help recall dozens of tainted supplements and alert consumers to hazardous products, he said.