The hepatitis C virus is responsible for more deaths in the United States than any other infectious disease, and hepatitis C infections have been rising at an alarming rate since 2010. We evaluated the role of the opioid epidemic and, in particular, the 2010 introduction of an abuse-deterrent version of OxyContin. The OxyContin reformulation led some users of the drug to switch to heroin, which could have exposed them to the hepatitis C virus. We used difference-in-differences methods, using data for the period 2004-15, to assess whether states with higher rates of OxyContin misuse prior to reformulation-states where the reformulation had more impact-experienced faster growth in infections after the reformulation. States with above-median OxyContin misuse before the reformulation experienced a 222 percent increase in hepatitis C infection rates in the post-reformulation period, while states with below-median misuse experienced only a 75 percent increase. These results suggest that interventions to deter opioid misuse can have unintended long-term public health consequences.