Viagra is one of Pfizer's top drugs, with $2 billion in worldwide revenue last year. And it's the most counterfeited drug in the U.S., according to the company.

A 2011 study, in which Pfizer bought "Viagra" from 22 popular Internet pharmacies and tested the pills, found 77 percent were counterfeit. Most had half or less of the promised level of the active ingredient.

Viagra is appealing to counterfeiters because it carries a double whammy: It's expensive and it treats a condition with an "embarrassment" factor.

Crooks running the illegal online pharmacies brazenly explain their ultra-low Viagra prices â¿¿ often $1 to $3 a pill â¿¿ by claiming they sell generic Viagra.

Generics are copycat versions of brand-name prescription drugs. They can't be sold legally until after a drugmaker's patent, or exclusive right to sell a drug, ends. Generic drugmakers don't have to spend $1 billion or so on testing to get a new drug approved, so their copycat versions often cost up to 90 percent less than the original drug.

But there is no such thing as generic Viagra in the U.S. Pfizer has patents giving it the exclusive right to sell Viagra here until 2020 and for many years in other countries.

Many patients are unaware of that.

Dr. David Dershewitz, an assistant urology professor at New Jersey Medical School who treats patients at Newark's University Hospital, says erectile dysfunction is common in men with enlarged prostates, diabetes and other conditions, but most men are too embarrassed to discuss it.

He says well over half of his patients who do broach the issue complain about Viagra's price. Some tell Dershewitz that they go online looking for bargains because they can't afford Viagra.

"The few that do admit to it have said that the results have been fairly dismal," but none has suffered serious harm, he says.

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