Nov 12 (Reuters) - The U.S. Food and Drug Administration warned on Tuesday of a counterfeit dietary supplement for male sexual enhancement that could be particularly harmful to patients with diabetes, high blood pressure, high cholesterol and heart disease.

In a safety warning posted on its website, the FDA said the fake product is represented as "ExtenZe Maximum Strength" and looks similar to the actual product, ExtenZe, which is made by Monrovia, California-based Biotab Nutraceuticals Inc.

The FDA said its analysis showed that the counterfeit ExtenZe contains sildenafil, an active ingredient in various FDA-approved prescription medicines, including Pfizer's Viagra, for erectile dysfunction. ()

Biotab's ExtenZe does not contain sildenafil, which cannot be taken without a prescription. The counterfeit product is available online and elsewhere without a prescription.

Sildenafil may interact with nitrates -- found in some prescription drugs and often taken by men with diabetes, high blood pressure, high cholesterol, or heart disease -- and could lower blood pressure to dangerous levels, the FDA said.

In February last year, Biotab started a voluntary recall of two lots of ExtenZe products after being informed by the FDA that the lots contained "undeclared drug ingredients" that could potentially be dangerous. ()

The Canadian health regulator warned in June against various unauthorized and "dangerous" ExtenZe products found at a retailer in Calgary that could pose serious health risks.

The FDA said the fake product can be identified by lot number 0512058 and the expiration date, "EXP. May 16," stamped on the outer carton and embossed on the blister card.

Customers with such counterfeit ExtenZe should stop taking the supplement immediately and contact their healthcare professional, the agency advised on Tuesday.