About one-third of the counterfeit pills actually differed in size from the real thing, while 42 percent differed in color.
Fifty-eight percent had too much active ingredient, sometimes as much as 2.4 times more, while 3 percent had no active ingredient at all.
Some contained unapproved compounds intended to promote an erection.
Only one of the counterfeit drugs contained "proper active ingredients," the researchers stated. Some contained potential toxins, including mercury and lead.
Even genuine Viagra has risks, experts note, especially for men who take nitrates for chest pain. And there could be drug interactions with both real and fake ED drugs.
"All these drugs have side effects and that's probably the big reason why patients should be getting them through a physician," Morley said. "While these things may be cheaper, they potentially have much greater side effects."
"We would hope by now that men would be happy to talk to their doctor [but] fundamentally, people are still shy of this," he continued.
There's more on counterfeit medications at the World Health Organization.
SOURCES: John Morley, M.D., director, geriatrics, and acting director, endocrinology, Saint Louis University, Missouri; May 31, 2010, presentation, American Urological Association annual meeting, San Francisco
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