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CHICAGO -- Most of the sildenafil (Viagra) sold online is fake and contains far less of the active ingredient than the real thing, researchers said here.

In an analysis of pills from 22 different websites claiming to sell the drug, 77% of samples were counterfeit and contained only between 30% and 50% of the levels of active ingredient advertised on its label, Irwin Goldstein, MD, of San Diego Sexual Medicine in California, and colleagues reported during a poster session at the World Meeting on Sexual Medicine.

"The fastest growing drug class in the world is counterfeit drugs," Goldstein said. "It's not just the phosphodiesterase 5 (PDE5) inhibitors [like sildenafil], but they are the vast bulk. It's a huge problem."

Irwin said fake drugs have been a burgeoning problem largely because of their increased online availability. In 2010, worldwide sales of counterfeit drugs was estimated to be $75 billion, and in that year in the U.S., $5.6 million in illegal pharmaceutical products were seized, up from $2.1 million in 2005.

He noted that one in six Americans purchased prescriptions online last year, translating to some 36 million consumers who may have been exposed to fake drugs. That includes all fake drugs, not just the PDE5 inhibitors, but he noted erectile dysfunction [ED] drugs are a target of counterfeiters because of the stigma some patients feel about the disorder and have to purchase drugs in-person.

To assess the black market for sildenafil, Goldstein and colleagues at Pfizer Global Security ordered drugs from the top 22 websites that came up in searches for "buy Viagra" in March 2011.

None of the websites required a prescription, and 91% claimed to sell "generic Viagra," which is not yet authorized by the FDA.

The cost per tablet ranged from $3.28 to $33, and the postal origins of the purchases were most commonly Hong Kong, the U.S., and the U.K., with only a few shipments from Canada, India, and other parts of China.

Irwin noted that the four Internet pharmacies claiming to be Canadian didn't ship medication from Canada; most came from Hong Kong instead.

They found that of the 22 sample tablets examined, 77% were counterfeit, 18% were authentic, and one was an illegal generic.

The authentic medications shipped from the U.S. and the U.K. and were apparently diverted from their original market, he said. The illegal generic, on the other hand, which closely resembled real sildenafil, shipped from India.

The counterfeit tablets only contained between 30% and 50% of the labeled amount of the active ingredient, sildenafil citrate. Goldstein said the drugs are largely made by "illicit narcotic labs."

There were also clear visual clues that the counterfeit tablets weren't real, Goldstein said. The packaging was different, and the pills were darker blue, heavier, and thicker than normal.

They were also found to contain undeclared ingredients such as gypsum -- the main ingredient in drywall -- as well as commercial paint and printer ink, he said.

Fakes could also contain other harmful active ingredients, such as other antihypertensive medications: "That's when it gets dangerous," Goldstein said.

Hartmut Porst, MD, a private practitioner in Hamburg, Germany, who moderated the discussion during which the poster was presented, said the problem is rampant in his country and it's nearly impossible to prevent patients from ordering their ED drugs online.

"They say, 'I'm getting a good erection and I don't get any side effects, and I pay only 20% of the normal cost,'" Porst said. "How can you convince them otherwise?"

Goldstein responded that doctors should ask patients how they are obtaining their medications, and that emergency departments "seeing patients getting sick from these medications must ask where they are getting them from."

Jim Wook Kim, MD, of Korea University Medical Center, said his country's sexual medicine society has been running an anti-fake drug campaign for years and its research has shown that four of 10 patients who go to the doctor's office for ED treatment had tried a fake ED drug at one point in their lives.

Edgardo Becher, MD, PhD, president of the International Society of Sexual Medicine, said in an email that his organization has published a position statement against fake PDE5 inhibitors in the Journal of Sexual Medicine.

"It is a widespread problem, totally uncontrolled, and nobody knows what those pills contain," he said in an email to 番茄社区app.

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