Thirty-thousand packs of fake life-saving drugs may have been consumed by NHS patients, the BBC has learned.
A senior official from the government's medicines watchdog said the case must lead to tightening of the law.
The Medicines Health products Regulatory Authority (MHRA) issued four emergency drugs recall notices in June 2007 - its highest state of alert.
An MHRA official told BBC File On 4 30,000 out of a batch of 70,000 fake drugs were still unaccounted for.
Three medicines, Casodex (bicalutamide) for prostate cancer, Plavix (clopidogrel) which treats strokes and heart conditions and Zyprexa (olanzapine) to control schizophrenia, had to be removed from pharmacies and warehouses across the UK in the recall 18 months ago.
The MHRA triggered four Class One recalls - its highest alert - in the space of a few days after it discovered thousands of packs of counterfeit drugs had entered the NHS supply chain.
Mick Deats, its head of enforcement, told File On 4: "It's not something done lightly, in fact it was the first time Class One recalls were ever carried out in the UK."
He added: "On all of the recalls in 2007, the meds involved had between 50% and 80% of the correct pharmaceutical ingredient in them."
A UK wholesaler had spotted discrepancies in packaging of stock bought elsewhere in Europe, but by the time that was picked up, thousands of drugs had been distributed to chemist shops, doctors and hospitals.
Mr Deats said: "We think about 70,000 packs were in the batch of which we seized about 40,000. Thirty-two thousand went out to pharmacy level and some got to patients and a recall was conducted where some of those were recovered. "
"It ran into the thousands that came back. Nevertheless, a significant amount got out to patient level and that is why Class One recalls had to be conducted to try to receive, quarantine or seize as much of it as we could."
Asked what happened to the 30,000 packs unaccounted for, Mr Deats said: "They would have been consumed by the patients."
Mr Deats told the BBC: "It's extremely rare to see counterfeit medicines in the regulated supply chain. It's a tiny fraction of a proportion."
He said this was no consolation to those who had taken the drug. "That's why this case has been taken so seriously which, we hope, will lead to changes in the law in the UK and we'll take the offenders to court"
I wonder what the side-effects are on the fake Zyprexa?
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