Overdose deaths associated with prescription and illicit opioids increased to 33,091 last year, according to a new report from the Centers for Disease Control and Prevention (CDC). That number marks an increase of almost 5,000 deaths from the previous year, The Washington Post reports.

Deaths from heroin and synthetic opioids such as fentanyl were largely responsible for the increase, the article notes.

Overdose deaths involving prescription opioids increased only slightly. This suggests that efforts to reduce the misuse of these drugs may be having an impact, according to a White House news release.

“Prescription opioid misuse and use of heroin and illicitly manufactured fentanyl are intertwined and deeply troubling problems,” said CDC Director Tom Frieden. “We need to drastically improve both the treatment of pain and the treatment of opioid use disorders and increase the use of naloxone to reverse opioid overdose. We must also work collaboratively with our public safety partners to further reduce access to illicit opioids.”