Peter Gillespie, 64, from Hertfordshire, was part of a £4.7m plot to bring two million doses of counterfeit drugs from China to the UK.
He was convicted of conspiring to defraud pharmaceutical wholesalers, pharmacists and members of the public.
He was convicted by a jury at Croydon Crown Court.
By mimicking authentic, properly manufactured and tested medicines, Gillespie illegally infiltrated the regulated system designed to protect the public and pharmaceutical industry, the court heard.
The counterfeit medicines contained only a fraction of the correct dosage.
They included Zyprexa, a medicine to treat schizophrenia and bipolar disorder.
Andrew Marshall, prosecuting, said patients had been put at risk by his fraud.
"This case is considered to represent the most serious breach of the medicine control regime - it's the most serious breach that has happened in the EU," he told jurors.
"It has had far-reaching effects for the pharmaceutical industry, control mechanisms, patients and the confidence of the public."
The case arose from a £750,000 three-and-a-half-year investigation by the Medicines and Healthcare products Regulatory Agency (MHRA).
Mick Deats, head of enforcement, said after the trial that 72,000 packs of counterfeit medicine - with a retail value of £4.7m - penetrated the UK supply chain between December 2006 and May 2007.
Some 25,000 of these packs reached pharmacies and were given to patients.
The MHRA was able to seize 40,000 before they got to pharmacies, and 7,000 were recovered following recalls.
Although the drugs contained just 50% to 80% of the correct ingredients, Mr Deats did not believe they caused any fatalities or adverse reactions.
At Croydon Crown Court, Gillespie, of High Street, Bovingdon, was also found guilty of selling or supplying drugs without a marketing authorisation  between January 2006 and June 2007.
Jurors also found him guilty of a charge of acting as a company director while disqualified.
Mr Deats added the MHRA now hoped to seize anything Gillespie obtained through his scam in a proceeds of crime hearing.
Four other men - his brother Ian, 59, of The Green, Marsh Baldon, Oxfordshire; Richard Kemp, 61, of School Lane, Y Waen, Flint, north Wales; Ian Harding, 58, of Lower Westwood, Bradford-on-Avon, Wiltshire; and James Quinn, 70, of Virginia Park, Virginia Water, Surrey - were acquitted of all charges.
Another person connected to the case had already been convicted in the US.
A man has been jailed for eight years for his part in what has been described as the most serious fake medicine fraud in the European Union.