Investigators have seized £8.6 million worth of counterfeit and unlicensed medicines destined for the UK's black market in just ten days.
Erectile dysfunction drugs accounted for a third of the doses of medication seized, followed by slimming pills and powerful medicines like sleeping pills, tranquilisers and antidepressants.
The medicines watchdog shut down 1,891 websites that were selling fake or unlicensed drugs as part of the operation between May 11 and 21, and five suspects were arrested.
The vast majority of the medicines seized in the UK came from India (72%), while 11% came from China.
A total of 3.6 million doses of fake and unlicensed drugs were seized in Britain, including 1.2 million doses of erectile dysfunction medicine; 383,000 of slimming products and 330,996 of other drugs including sleeping pills, tranquilisers and antidepressants.
The Medicines and Healthcare Products Regulatory Agency (MHRA) took part in the international crackdown, dubbed Operation Pangea, that was run through Interpol.
It has also targeted videos posted on YouTube promoting the sale of fake medicines and has removed 18,671 clips from the site in the past year.
MHRA's head of enforcement, Alastair Jeffrey, said: " The medicines recovered during these raids were being held in appalling conditions, such as a dirty old building with broken windows, with medicines lying on the floor in bin bags.
"Criminals involved in the illegal supply of medicines through the internet aren't interested in your health, they are interested in your money, whether they get this through selling you a potentially dangerous counterfeit or unlicensed drug or through stealing your bank details.
"To protect your health, visit your GP, get a correct diagnosis and buy medicines from a legitimate high street or online pharmacy."
At the end of last year, it was claimed that as many as 1.5 million people are addicted to prescription drugs in the UK, far more than are being treated for addiction to illegal drugs.
The Home Affairs Select Committee suggested that GPs should start collecting anonymous data on patients who are hooked on prescription drugs to establish the true scale of a problem that is lurking "in the shadows".