Injection drug users (IDUs) in the United States shooting up prescription opioid painkillers are five times more likely to have the hepatitis C virus (HCV) than those who inject street drugs like heroin or cocaine, according to a Huffington Post interview with the lead author of a new study.

Published in the American Journal of Public Health, the study sheds light on the nation’s rapidly growing hep C epidemic among young IDUs, many of whom are becoming addicted to injection drugs via the influx of pain pills across the country for the past 20 years.

Jon Zibbell, PhD, the lead author and a researcher at the division of viral hepatitis at the Centers for Disease Control and Prevention (CDC), said scientists were unsure exactly why pain pills were associated with increased infection risk, but they had some hypotheses.

Many prescription opioid pills, especially those made with abuse-deterrent formulas, don’t draw into a syringe easily without adding large amounts of water. This often requires the drug user to either inject multiple times to get a full drug dose or use syringes with larger barrels (and subsequently more dead space) to shoot up—both of which increase a person’s risk of contracting a blood-borne infection like HCV.

It also takes much longer to prepare prescription painkillers for injection than drugs like heroin. Researchers believe that the increased time may increase the chances of contamination as well. The study also found that people who shared injection equipment, such as cookers, cottons and water used to prepare the drugs, were four times more likely to become HCV positive than those who didn’t.