More than 50 percent of U.S. adults take vitamins, herbs or supplements but many may not know some contain prescription drugs or toxic metals, a doctor says.

Dr. Jose Luis Mosquera, medical adviser to Consumer Reports, said some supplements are really prescription drugs.

Many recalled products have the same or similar active ingredients as prescription drugs such as sildenafil, or Viagra; tadalafil, or Cialis; and sibutramine, or Meridia -- a weight-loss drug that was removed from the market in 2010 because of evidence it increased the risk of heart attacks and strokes, the consumer magazine said.

"Adulterated products can cause some of the same side effects and interactions that a consumer may be trying to avoid by opting for supplements instead of prescription drugs," Mosquera said.

Some people can overdose on vitamins and minerals and most probably don't take more than 100 percent of the recommended daily allowance of a particular nutrient unless prescribed by a physician. Megadoses of the fat-soluble vitamins A, D, E, and K could cause problems, and even standard doses might interfere with some prescription medicines, Mosquera said.

"Surprisingly, some people may experience adverse effects from even normal doses of a vitamin or mineral supplement, especially patients with digestive issues or those who take blood thinners," Mosquera said.

The article, published in the September issue of Consumer Reports and online at www.ConsumerReports.org, advised, "Lay off the antioxidant supplements and reduce your cancer risk safely by quitting smoking, avoiding excessive alcohol, and eating a healthy diet that includes plenty of fruit, vegetables, nuts, legumes, and whole grains."