When a dietary supplement sounds too good to be true, it probably is.

The U.S. Food and Drug Administration is warning consumers that Magic Power Coffee, an instant coffee product marketed as a dietary supplement for sexual enhancement, contains an active drug ingredient that can dangerously lower blood pressure.

If you have any of the stuff, "stop using it immediately," the FDA warned this weekend.

The agency found Magic Power Coffee contains hydroxythiohomosildenafil. If the long name alone isn't enough of a turnoff, here's the science you should know: It is a chemical similar to sildenafil, the active ingredient in Viagra. But hydroxythiohomosildenafil, like sildenafil, may interact with prescription drugs known as nitrates, including nitroglycerin, and cause dangerously low blood pressure, an FDA statement notes.

When blood pressure drops suddenly, the brain is deprived of an adequate blood supply, which can lead to dizziness or lightheadedness.

"Because this product is an instant coffee 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 fact, Magic Power Coffee can cause serious harm."

To date, the FDA is not aware of any adverse events associated with the use of this product, however. The stuff is sold at Internet sites and online auctions by multiple independent distributors participating in an online multi-level marketing scheme (another hint that you might want to avoid the whole scheme).

If you note any side effects of the product, see a doctor, the FDA suggests. Consumers and health care professionals should report adverse events to the FDA's MedWatch program at 800-FDA-1088 or online at www.fda.gov/medwatch/report.htm.