WASHINGTON ? The US food and drug safety watchdog warned Thursday that an over-the-counter men's sex aid, labeled as all-natural, contains a chemical similar to the active ingredient in Viagra and could be dangerous.

"'Stiff Nights', a product marketed as a dietary supplement for sexual enhancement, contains an ingredient that can dangerously lower blood pressure and is illegal," the Food and Drug Administration (FDA) said in a statement.

The FDA began probing Stiff Nights after receiving a customer complaint about the product. The agency did not reveal the nature of the complaint.

The investigation found that rather than being all-natural, Stiff Nights contains sulfoaildenafil, a chemical similar to the ingredient in Viagra.

"Because this product is labeled as an all-natural dietary supplement, consumers may assume it is harmless and poses no health risk," said Deborah Autor, director of the FDA?s Center for Drug Evaluation and Research Office of Compliance.

"In fact, this product is illegally marketed and can cause serious complications," she said.

Sulfoaildenafil could interact with prescription drugs known as nitrates and cause dangerously low blood pressure.

Stiff Nights is sold online and at retail stores in bottles or blister packs containing red capsules.

A bottle of 12 Stiff Nights capsules was being offered Thursday on Amazon for around $27.

Another website -- mystiffnights.com -- said the product is made with "only the finest herbs and greens."