Updated

Federal drug safety officials are warning consumers about counterfeit sex-enhancement pills being sold without a prescription but containing the drugs used in Viagra and a similar medication.

The bogus pills are sold as ExtenZe, an herbal supplement, and resemble real ExtenZe pills, the Food and Drug Administration said Friday.

Yet they contain tadalafil and sildenafil, the active ingredients in Cialis and Viagra, the agency said. Both drugs require a doctor's prescription.

The counterfeit products are marked with lot numbers 1110075 and F050899, the FDA said. It said consumers should stop taking questionable pills and contact their doctors about any side effects.

ExtenZe is manufactured by Biotab Nutraceuticals Inc., according to the company's website. FDA spokeswoman Shelly Burgess confirmed that the fake pills were not manufactured by Biotab.

This is the latest in a string of cases where real drugs for erectile dysfunction and other conditions appeared in herbal supplements that were marketed to treat those conditions.

In February, Biotab voluntarily recalled two lots of counterfeit ExtenZe that contained tadalafil, sildenafil and sibutramine, a weight-loss pill that is not approved for sale in the U.S.

At the time, Biotab said that other counterfeit products might still be on store shelves.

"This incident is an example of a growing trend of products marketed as dietary supplements or conventional foods with hidden drugs and chemicals," said Ilisa Bernstein, deputy director of the Office of Compliance in the FDA's Center for Drug Evaluation and Research.

"These types of products are typically promoted for sexual enhancement, weight loss, and body building, and are often represented as being 'all natural.' Consumers should exercise caution before purchasing products promoted for these uses," she said in a statement.

The FDA said in December that it is cracking down on supplements containing prescription ingredients.

From 2007 to 2010, the FDA pressured companies to recall nearly 200 inappropriately formulated products, the agency said in December. The recalled products were linked to reports of stroke, kidney failure, liver injury and death.

With the exception of infant formula, the FDA does not have the authority to order a recall of a food or dietary supplement. It usually issues warning letters to draw attention to illegal products.

Dietary supplements can slip through the regulatory cracks because they can be marketed without FDA approval. Manufacturers are responsible for ensuring that the products are safe.

In a letter to the supplement industry, the agency said that manufacturers who distribute tainted products could face criminal prosecution.

The FDA did not identify the maker of the fake ExtenZe pills. Citing agency policy, FDA spokeswoman Burgess declined to say whether the agency is pursuing criminal charges.

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Associated Press Business Writer Josh Freed in Minneapolis contributed to this report.