A growing number of people are dying from cocaine-related overdoses because they are mixing the drug with opioids such as heroin and fentanyl, according to U.S. News & World Report.

Cocaine can result in overdose on its own, the article notes. It is not known whether people are mixing the drugs intentionally, or are unknowingly taking tainted products.

According to the Centers for Disease Control and Prevention, cocaine was responsible for the second-most drug-overdose deaths in 2014. Cocaine-related deaths have risen in recent years, after declining steadily, even though there did not appear to be a significant increase in the drug’s availability.

“From the death data, we don’t know whether these are cocaine users who added opioids or were opioid users who added cocaine,” said Dr. Wilson Compton, Deputy Director of the National Institute on Drug Abuse. “Both are possible. The data shows us that both drugs may have been related to the deaths.”