Ashish Halai, 33, ordered fake Viagra tablets from suppliers in China and Mexico for 25p each and sold them for as much as £20 online to people in the US and Europe who were too embarrassed to go to their doctor.
Three other members of the gang were also found guilty and will be sentenced later.
The case came to light after an investigation by the Medicines and Healthcare products Regulatory Agency (MHRA), which estimated that the gang netted at least £2m from the scam.
Mick Deats, head of enforcement at the MHRA, said: "The MHRA treats every report of a counterfeit medicine as a serious incident. This successful prosecution should serve as a clear signal to those contemplating the supply of counterfeit medicines.
"The public are strongly advised to avoid buying medicines online, where the risk of being provided with counterfeit medicines is greatly increased."
Halai is a chemist who sold his practice in Bayswater, London but continued to use the name to sell herbal supplements.
The court heard that Halai began selling fake anti-impotence drugs in 2002 passing them off as Viagra and Cialis. He packaged the drugs so skilfully that experts said it would take a trained eye to spot the difference.
Police seized over £1.5m worth of the fake drugs when they swooped on the gang. The drugs were imported using business courier services and were disguised as pet supplements.
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