Some sexual enhancement products sold on the Internet contain the same ingredients as prescription drugs such as Viagra and are not only illegal but dangerous, the US Food and Drug Administration cautioned on Wednesday.

Erectile dysfunction products can cause a deadly interaction with many heart drugs, notably those containing nitrates.

"The US Food and Drug Administration is warning consumers not to purchase or consume Zimaxx, Libidus, Neophase, Nasutra, Vigor-25, Actra-Rx, or 4EVERON," the agency said in a statement.

"These products threaten the public health because they contain undeclared chemicals that are similar or identical to the active ingredients used in several FDA-approved prescription drug products," added Dr. Steven Galson, Director of the FDA's Center for Drug Evaluation and Research.

"This risk is even more serious because consumers may not know that these ingredients can interact with medications and dangerously lower their blood pressure."

Zimaxx contains sildenafil, the active pharmaceutical ingredient in Viagra, Pfizer's prescription erectile dysfunction product.

The other products contain chemical ingredients that are similar either to sildenafil or to vardenafil, the active ingredient in Levitra. Levitra is a prescription erectile dysfunction drug developed and sold jointly by Bayer AG, GlaxoSmithKline Plc and Schering-Plough Corp.

"Consumers with diabetes, high blood pressure, high cholesterol, or heart disease often take nitrates," the FDA said.

"ED (erectile dysfunction) is a common problem in men with these conditions, and they may seek products like the ones noted above because these products claim that they are 'all natural' or that they do not contain the active ingredients used in FDA-approved ED drugs."

The agency said it had stopped a shipment of 4 EVERON from entry to the United States and warned the companies selling the drugs.