Pharmaceutical experts warned that the rapid growth of Internet commerce has led to an explosion of counterfeit drugs being sold worldwide. The experts have pointed China as the biggest source of fake medicines.
The illicit trade is now believed to be worth around 75 billion dollars globally, with criminal gangs increasingly using the web to move their products across borders, said Scott Davis, Pfizer's top security expert for Asia.
"The Internet has led to an explosion of availability of these products," Davis told a health forum in Manila.
"About 90 percent of counterfeit drugs... are at some point marketed and sold on the Internet."
He said websites selling fake drugs commonly did not have physical addresses and exploited weak or murky customs regulations to ship their products.
"They are getting more sophisticated," he said, adding that law enforcers were finding it ever more difficult to tell the fake drugs from the real.
"We often have to send the pills to our labs to tell us the difference," he said.
The US pharmaceutical giant Pfizer makes the anti-impotence drug Viagra, which along with Eli Lilly's Cialis are among the world's most widely copied drugs.
"But now it's not just Viagra or lifestyle drugs but other medicines like malarials," he said. "These criminals will copy anything to make a buck."
He said China was the number one source of counterfeit drugs last year, followed by Jordan, the United States, Israel and Canada.
Catherine Dauphin, a World Health Organization expert on pharmaceutical policy, told the same forum that more than half the drugs sold on Internet sites without approval from governments were fake.
She said criminal gangs typically lured the public into buying fake drugs by offering them at cheaper rates and without the need for medical prescriptions.