The US Food and Drug Administration on Tuesday warned consumers not to take a product called "Japan Rapid Weight Loss Diet Pills" because they contain a suspected cancer-causing agent.
The pills, advertised as promoting weight loss, are distributed by a company called Xiushentang and sold on popular web sites including Amazon.com, said the FDA.
Consumers are advised to stop using the pills -- which are packaged in three colors: green, yellow and blue -- and discard them. They should also see a doctor if they have experienced any side effects, the agency said.
An FDA lab analysis has confirmed that the pills, "Japan Rapid Weight Loss Diet Pills Green" contain "undeclared phenolphthalein," the agency said in a statement.
"Phenolphthalein is a solution used in chemical experiments and a suspected cancer-causing agent that is not approved for marketing in the United States."
The FDA has classified the agent as "not generally recognized as safe and effective," and it has been found to potentially damage or even cause mutations in a person's DNA, the warning said.
A similar notice was issued for "Japan Rapid Weight Loss Diet Pills Yellow" because they contain both phenolphthalein and sibutramine, a weight loss agent found in the drug Meridia that was removed from the market in 2010 because it boosted the risk of heart attack and stroke.
"The product poses a threat to consumers because sibutramine is known to substantially increase blood pressure and/or pulse rate in some patients and may present a significant risk for patients with a history of coronary artery disease, congestive heart failure, arrhythmias or stroke," the FDA said.
A third product, "Japan Weight Loss Blue" contains sibutramine and ephedrine alkaloids. Products containing ephedra have been banned because they can boost blood pressure to risky levels.
The agency said the dangerous weight loss pills are part of an emerging trend in which companies sell products purporting to be dietary supplements but which contain hidden ingredients that can be harmful.