Generic PrEP purchased online is being credited with plunging rates of new HIV infections among gay men in London.

According to New Scientist, four London sexual health clinics saw dramatic drops in new cases of around 40 percent last year compared to 2015.

Experts theorize that an underground network of cheap generic PrEP drugs may have contributed to the decline.

Will Nutland at the London School of Hygiene and Tropical Medicine has created a website called PrEPster that informs people about how to acquire PrEP online.

“We need to be very cautious at this stage, but I can't see what else it can be,” Nutland told New Scientist. “Something extraordinary has happened in the last 12 months because of a bunch of DIY activists working off our kitchen tables.”

While PrEP has been approved in the UK to prevent HIV infection, it is currently not available on the National Health Service. The brand-name drug Truvada can be purchased privately but is expensive. Instead, generic versions of the drug from India and Swaziland are being purchased online. Such transactions are frowned upon, because the process could be illegal or the drugs may not be safe.

However, New Scientist reports that no pills have yet to be found to be fake.