In 2013, more than 16,000 Americans overdosed and died with prescription painkillers in their systems. That number would grow in the years to come, but already it was alarming, more than 400 percent higher than the same statistic for the year 1999.

And so, in October of 2014, the Drug Enforcement Administration changed its policies around some of the most commonly prescribed opioids—including Vicodin and Lortab—making it more difficult for doctors to prescribe them and for patients to get refills. Almost immediately, prescriptions went down. But did the policy change have unwanted consequences too?

In a new study, a team of social scientists finds evidence that the DEA change may have caused illicit sales of prescription opioids to increase instead. The study is an important look at whether a policy aimed at reducing the drug supply works to lower drug use—an area that’s understudied, as one pair of researchers recently argued. (More often, scientists study tactics aimed at reducing demand, such as anti-drug campaigns.)

The new research, published Wednesday in the journal BMJ, can’t prove cause. It’s possible that pill demand grew for other, unrelated reasons. Still, the researchers found a few reasons to believe the DEA policy contributed: During the same time frame, sales of other drugs on the dark Web sites that the scientists analyzed didn’t go up, and pill sales only went up among sellers in United States, not in other countries.

To conduct the study, four researchers from Australia, the United Kingdom, and Canada deployed software called DATACRYPTO on 31 of the world’s largest dark Web marketplaces operating immediately before and after the DEA enforcement change. Dark Web marketplaces are encrypted websites where people can anonymously buy illegal material, including drugs and unlicensed guns. DATACRYPTO crawled these sites, harvesting data on what types of drugs were on offer, the drugs’ countries of origin, and the number of customer comments on each seller’s page, which researchers used as a proxy for how much product that seller sold. The research team looked at sales of prescription opioids, sedatives, stimulants, and steroids, as well as heroin. The only statistically significant change in sales they found immediately after 2014 was of prescription opioids sold in the U.S.

The U.S. government knows that a certain slice of Americans get their drugs online. In January, the Department of Justice announced the creation of a unit dedicated to taking down dark Web opioid and cocaine sellers; in April, it announced the unit’s first arrests. How such enforcement will affect the overall market and drug use in the U.S. remains to be seen.