A new study links the reformulation of OxyContin to an increase in the spread of hepatitis C.

The company that makes OxyContin, Purdue Pharma, reformulated the drug to make it more difficult to crush or dissolve. The change made it harder for people to snort or inject the drug. As a result, many people who had been addicted to OxyContin turned to heroin, according to HealthDay. People who inject heroin often share needles, which can spread hepatitis C.

The study found states with above-average rates of OxyContin misuse before the drug was formulated saw hepatitis C infections rise three times as fast as in other states. Hepatitis C causes liver disease, and was responsible for 20,000 deaths in the United States in 2015.

“These results show that efforts to deter misuse of opioids can have unintended, long-term public health consequences,” David Powell, the study’s lead author and a senior economist at the nonprofit research organization RAND, said in a news release. “As we continue to develop policies to combat the opioid epidemic, we need to be careful that new approaches do not make another public health problem worse.”