Hundreds of dodgy drugs were seized by officers from a medical watchdog in a raid in the borough which was part of an international operation.
The Medicines and Healthcare Products Regulatory Agency (MHRA) swooped on a house in Rothesay Avenue, Greenford.
They found 300 packs of a herbal slimming pill on Wednesday last week which were ready to be sold online.
It was one of 12 raids across the country as part of Operation Pangea, the first co-ordinated international series of seizures of unlicensed or prescription-only medicines.s
Other strikes were made in Canada, Germany, Ireland, New Zealand, Singapore, Switzerland and the US.
The slimming pills found in Greenford are called Sida Cordifolia and are banned unless sellers have specific permission from the MHRA, which controls their use.
They were among a thousand packs of illegally-sold medicines found across the country. A number of websites have been shut down.
Danny Lee-Frost, head of operations at the MHRA, said: "People can be at considerable risk if they buy medicines from illegal websites.
"Our actions should serve as a stark warning to those who are engaged in any way with supplying medicines illegally."
* To report counterfeit medicines email the MHRA on firstname.lastname@example.org or call the 24-hour hotline on 020 7084 2701.