The Food and Drug Administration is warning consumers to stay away from dietary supplements that claim to promote sexual enhancement, weight loss or body building because they may contain hidden — and potentially dangerous — ingredients.

Such products are sold without a prescription on various websites, including Amazon, and possibly in some retail stores, according the FDA.

But often, consumers aren't getting what they paid for.

"Very often, the supplement pills don’t even have in them what they say on the bottle,” Dr. Daniel Shoskes, a urologist at the Cleveland Clinic, said.

Worse, they could be spiked with pharmaceuticals that can have dangerous interactions with other medications, he added.

This week, the FDA issued public notifications on two products marketed for male sexual enhancement, called Man Fuel Xtreme Edition and Man Fuel Male Enhancement Shooter (Tropical Fruit Flavor).

The Food and Drug Administration is advising consumers not to purchase or use Man Fuel Male Enhancement Shooter (Tropical Fruit Flavor). FDA

"FDA laboratory analysis confirmed that Man Fuel Xtreme Edition contains sildenafil, dithiodesmethyl carbodenafil and desmethyl carbodenafil. Sildenafil is the active ingredient in the FDA-approved prescription drug Viagra, used to treat erectile dysfunction," one notification said.

And the FDA said that its tests of Man Fuel Male Enhancement Shooter found that the product contains tadalafil, the active ingredient in Cialis, another FDA-approved medication to treat erectile dysfunction.

Both sildenafil and tadalafil are known to interact with compounds called nitrates, found in some prescription drugs, including those used to treat high blood pressure or chest pain.

Drugs such as Viagra and Cialis work in a similar way to nitrates: They cause the muscles that control blood vessels to relax. Working together, they could lead to a dangerous drop in blood pressure.

Shoskes told NBC News his patients ask him about so-called herbal supplements "all the time." He warns self-medicating erectile dysfunction is not a good approach.

"Erectile dysfunction is a condition that should prompt an appointment with a physician," he said. "It can be an initial red flag for other problems, like heart disease, in the body."

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