The Centers for Disease Control and Prevention (CDC) is reporting a 426 percent increase in seized drugs that tested positive for fentanyl between 2013 and 2014, according to NPR. The number of deaths involving fentanyl and other synthetic opioids increased 79 percent during that period.

The CDC analyzed data from 27 states, and found a strong link between increases in synthetic opioid deaths and seized fentanyl products, but not with changes in fentanyl prescribing. The findings suggest that illegally made fentanyl is behind the increase in overdoses, according to the CDC’s Morbidity and Mortality Weekly Report.

Fentanyl is prescribed to treat severe pain. The fentanyl that is being mixed with heroin and sold on the streets is being illicitly manufactured, the CDC noted.