Increased regulation of opioids has led to a growing number of sales online, through an underground network of websites known as the “darknet,” Newsweek reports.

In 2014 the Drug Enforcement Administration (DEA) changed the designation of opioids such as oxycodone and hydrocodone from Schedule III to Schedule II, which means doctors have more limitations on prescribing them.

Researchers analyzed darknet sales for opioids in the year before the DEA made the change and in the two years afterwards. They found a significant increase in American opioid sales after the change. In contrast, they found no significant change in sales of other drugs or in other countries where there was no schedule change. The findings are published in BMJ.

“We’re not seeing a ‘spike,’” said study author James Martin. “This isn’t a temporary up and down. We’re seeing a permanent sustained increase.”