In October 2014, hydrocodone combination products – the most popularly prescribed opioid pain relievers – were reclassified from the U.S. Drug Enforcement Administration’s schedule III to schedule II, imposing stricter controls on prescriptions written by doctors and on patients’ ability to refill them.

And according to a study published this month in the British Medical Journal, almost immediately, the proportion of all drugs illicitly purchased in the U.S. from sellers on the “dark net” that were in the opioid category began rising, reaching 13.7 percent in 2016. Stronger, more dangerous opioids also gained in popularity in these so-called cryptomarkets, including fentanyl, which went from least common to the second most commonly purchased opioid.

“Our dataset covered the period of the federal hydrocodone re-scheduling and, as a researcher interested in causal analysis, the initial approach was to see if we’d observe what we thought we’d observe – i.e. an increase (in cryptomarket sales) of prescription opioid set against no changes elsewhere,” one of the authors said to Reuters Health.

The researchers used special software to analyze drug sales in 31 of the world’s largest cryptomarkets operating from October 2013 to July 2016. They focused on six product types: prescription opioids, prescription sedatives, prescription steroids, prescription stimulants, other prescription drugs and illicit opioids such as heroin.

Based on sales before and after the opioid rescheduling, they saw no change in purchases of opioids from outside the U.S., and no changes in any other drug categories. But their model projected that opioids would have represented 6.7 percent of all drugs purchased in the U.S. from cryptomarkets without the schedule change. Instead, opioid market share was more than twice that by July 2016.

Indeed, the study team also found that illegal sales of the more potent opioids, oxycodone and fentanyl, increased by the greatest amounts in these cryptomarkets.

“The Martin et al study adds to a mounting body of evidence that multiple efforts to restrict access to prescription opioids coincided with efforts by many opioid users to access these drugs on the black market,” said Leo Beletsky, an associate professor of law and health sciences at Northeastern University in Boston, who co-authored an editorial accompanying the study.

The emergence of cryptomarkets during this time functioned as “disruptive innovation” in how people can obtain drugs outside legitimate channels, Beletsky said in an email.