In 2010, heroin was involved in 8% of U.S. drug overdose deaths, a study by the Atlanta-based center said. By 2015, that proportion had jumped to 25%.

The center’s research was based on death certificate data and did not examine the underlying causes. But a 2015 study by the U.S. Centers for Disease Control and Prevention found that the declining price of heroin and its increasing purity might be causing more people to use it.

The latest statistics come as authorities in several parts of the United States grapple with opioid and heroin crises.

Experts say there is a link between heroin use and much higher rates of prescription opioid painkiller usage.