More than half of the herbal and dietary supplements analyzed by researchers contained ingredients that differed from the list on their labels.
Some had hidden ingredients that might actually harm health, researchers said.
Bodybuilding and weight-loss supplements, in particular, tended to contain ingredients not listed on their packaging, said lead researcher Dr. Victor Navarro, chair of hepatology for Einstein Medical Center in Philadelphia.
Chemical analyses found that product labels did not reflect ingredients for 80 percent of bodybuilding and performance enhancement supplements, and 72 percent of weight-loss products, the researchers reported.
"We found that half of the bodybuilding supplements in our analysis contained undeclared anabolic steroids," Navarro said.
The researchers and health experts are concerned that these mystery ingredients can cause lasting liver damage.
More than 20 percent of liver damage cases reported to the U.S. Drug-Induced Liver Injury Network are attributed to herbal and dietary supplements, the researchers said in background notes.
Sonya Angelone is a spokesperson for the Academy of Nutrition and Dietetics. She said, "Your liver is your major detoxification organ. That's why you're going to see liver problems with these products." Angelone, a San Francisco registered dietitian, wasn't involved in the new study.
Navarro and his team analyzed more than 200 supplements reported to the liver injury network by hundreds of patients, to see whether their labels reflected the actual contents.
Only 90 of 203 products had labels that accurately reflected their content, the investigators concluded.