In a Thursday statement, the US Food and Drug Administration (FDA) has warned the consumers against the use of 'Stiff Nights' - a supposedly all-natural over-the-counter dietary supplement for sexual enhancement - saying that the product contains an active Viagra ingredient.

'Stiff Nights', available in bottles containing 6, 12, or 30 red capsules or in blister packs containing one or two capsules, is distributed on Internet sites and at retail stores by Impulsaria LLC of Grand Rapids, Michigan.

Despite the fact that 'Stiff Nights' is marketed as an all-natural sex pill for men, FDA has clarified that the presence of the chemical ingredient in the pill is illegal and "can dangerously lower blood pressure" upon its interaction with prescription drugs called nitrates, including nitroglycerin.

The FDA warning about the 'Stiff Nights' pill follows its investigations initiated after receiving a customer complaint about the product. Though the agency did not elaborate on the nature of the complaint, it has found that the product contains sulfoaildenafil, which is quite akin to Viagra's active ingredient, sildenafil.

Commenting on 'Stiff Nights,' Deborah M. Autor, director of FDA's Center for Drug Evaluation and Research Office of Compliance, said: "Because this product is labeled as an 'all natural dietary supplement,' consumers may assume it is harmless and poses no health risk. In fact, this product is illegally marketed and can cause serious complications."