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An instant coffee that promises instant erections has been criticised by health experts.

Stiff Bull claims a cupful of the exotic herbs-packed drink will make guys “stay harder longer.”

It says on its website: “The coffee contains a propriety blend of ingredients such as - tongkat Ali, maca root, and guarana.

“These herbs grow wild in the jungles of Malaysia and have been used for centuries by the people of Asia and South America to greatly improve sexual health, libido, and overall wellness.

“The combination of these ingredients produces an awesome effect that can last up to 2 - 3 days.”

But Maryland-based Stiff Bull - marketed as The Relationship Saver - has suffered a set back to its launch after America’s Food and Drugs Agency added it to its list of Tainted Sexual Enhancement Products.

It warns consumers should not to drink it because it contains an unlisted drug used for erectile dysfunction.

It could save some relationships but could be dangerous if unknowingly used with some other medications.

The coffee includes desmethyl carbodenafil, a chemical similar to the active ingredient in Viagra but is not listed on the packaging, says the FDA.

It warned: "This undeclared ingredient may interact with nitrates found in some prescription drugs such as nitroglycerin and may lower blood pressure to dangerous levels.”

It could be especially dangerous to men with diabetes, high blood pressure, high cholesterol, or heart disease who often take nitrates.

Stiff Bull was identified by the FDA during an examination of international mail shipments.

It is the 34th product the agency has warned consumers about this year following a string of sex improvement products were found to include undeclared ingredients.