Between 2013 and 2016, the U.K.’s Medicines and Healthcare Regulatory Agency estimates that £115 to £200 million ($161 to $280 million) of medicines were taken from the legitimate supply chain and sold on criminal markets, endangering thousands of people and prompting agency investigations.

More than 40 arrests were made in MHRA’s active investigations during 2017, which revealed an extensive criminal network that involved wholesale dealers in addition to a small number of registered pharmacies diverting medicines.

The agency’s investigations are focused on prescription-only drugs such as benzodiazepines, anxiolytics and painkillers like Tramadol, which are being sold online illegally and can result in addiction because they are not properly prescribed by a licensed physician.