Erectile dysfunction drugs accounted for a third of the doses of medication seized, followed by slimming pills and powerful medicines like sleeping pills, tranquillisers and antidepressants.
The medicines watchdog shut down 1,891 websites that were selling fake or unlicensed drugs as part of the operation from May 11-21, 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, tranquillisers 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.
The 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."