The Food and Drug Administration says 72 different supplements sold in stores and on the Internet are filled with hidden dangers.
The original warning was issued late last year, but recently the FDA expanded its list of dangerous products. The agency is warning consumers about high levels of prescription medications hidden in over-the-counter products.
Doctors at UT Southwestern Medical Center said dieters may take the supplements to feel healthy, but the FDA says every supplement on their list carries serious risks.
"You may be getting more than you need, or you may be getting an extra drug that you didn't know was in there, which could interact with one of your other medications or a health problem," said Dr. Eve Guth.
Some of the supplements claim to be "natural" or contain only herbal ingredients, but actually have hidden drugs not approved for use in the United States.
"[The label] might say it has 10 milligrams of substance 'X,' but when they take it into the lab and test, it might have 2 milligrams or it might have 32 milligrams," Guth said. "Now that might not be a problem, or it could be something very dangerous."
FDA tests found antidepressants, antiseizure medicines, potent diuretics and one solution used in chemical experiments that's a suspected cancer-causing agent and not approved for use in the United States.
Sibutramine, for example, a controlled substance, was found in many supplements at levels much higher than the maximum daily dosage for Meridia. Meridia is the only FDA-approved drug product containing sibutramine. The health risks can be very serious and can cause high blood pressure, heart attack, seizures and stroke.
"I think you're better served using the products that are from established companies," Guth said.
"If its too good to be true, it isn't true," Walkup said.
The FDA has a complete list of the 72 weight loss supplements containing tainted products on their web site.