The U.S. Food and Drug Administration is warning consumers that "Stiff Nights," a product marketed as a dietary supplement for sexual enhancement, contains an ingredient that can dangerously lower blood pressure and is illegal.

Following a consumer complaint, the FDA determined that Stiff Nights contains sulfoaildenafil. This is a chemical similar to sildenafil, the active ingredient in Viagra. Sulfoaildenafil may interact with the prescription drugs nitrates, including nitroglycerin, and cause dangerously low blood pressure.

The product is distributed online and at retail stores by Impulsaria LLC of Grand Rapids, Mich. It is sold in bottles containing 6, 12, or 30 red capsules or in blister packs containing one or two capsules.

"Because this product is labeled as an ‘all natural dietary supplement,’ consumers may assume it is harmless and poses no health risk," said Deborah M. Autor, director of FDA’s Center for Drug Evaluation and Research Office of Compliance, in a news release. "In fact, this product is illegally marketed and can cause serious complications.”

The FDA advises consumers who have experienced adverse effects from sexual enhancement products to consult a health care professional.

Consumers and health care professionals should report adverse events to the FDA's MedWatch program at 1-800-FDA-1088 or online at www.fda.gov/medwatch/report.htm.