Two pharmacists in Yorkshire have been arrested after it was found they had purchased more than 200,000 packs of benzodiazepines and other "powerful" drugs but only dispensed very small amounts.
The arrests were the result of the UK drug regulator's large-scale investigation into the diversion of prescription drug supplies into the criminal market.
The Medicines and Healthcare Products Regulatory Agency (MHRA) said that, uncommonly, the drugs involved were legitimate UK products.
It said the investigation has found an "extensive network of criminality involving businesses such as wholesale dealers and including a small number of registered pharmacies in the UK".
Alastair Jeffrey, MHRA head of enforcement, said: "Selling medicines other than through the legitimate supply chain is illegal. Evidence shows extensive criminality involving a number of businesses.
"As a matter of priority we are working with our regulatory and law enforcement colleagues to identify and prosecute those involved in this activity and to implement preventative measures to make sure this illegal industry is shut down.
"Prescription only medicines are, by their very nature, potent and should only be prescribed by a doctor or appropriate healthcare professional. We would advise people not to buy medicines from unregulated sources as they pose a danger to their health," he said.