Opioid overdoses are climbing. But prescription painkillers aren’t driving them anymore.

The crackdown on opioid prescriptions to rein in the raging epidemic of opioid abuse and overdoses is picking up steam. Ten states have passed legislation that limits new opioid prescriptions to 10 days or less (in line with 2016 Centers for Diseases Control and Prevention guidelines), and more states are likely to follow suit. This week, President Trump signed an executive order that would create a commission to review various strategies to prevent addiction.

But as prescriptions for oxycodone and morphine get more restrictive, opioids sold on the black market are eclipsing them as a bigger threat, at least when it comes to overdoses.

More than 33,000 people died from opioid drug overdose in 2015 — the highest number of opioid-related deaths since at least the late 1990s. But for the first time, in 2015 more people died from heroin than prescription painkillers such as hydrocodone and oxycodone.

Incredibly potent synthetic opioids like fentanyl are also responsible for more drug overdose deaths. Deaths from fentanyl more than doubled in 2014, from 1,905 deaths in 2013 to 4,200 deaths.

“What’s happening now is the number of prescription opioid overdoses are stabilizing,” said Chinazo Cunningham, a professor of internal medicine at Albert Einstein College of Medicine who helped review the 2016 CDC guidelines on opioid prescriptions. “But opioid overdose rates have not plateaued, because heroin use is dramatically increasing.”

Data from states and cities also indicate that since 2010, drug overdoses from prescription opioids have more or less stabilized while deaths from heroin and fentanyl have skyrocketed.

For instance, a 2016 paper analyzing data from Jefferson County, the most populous county in Alabama, found that drug overdose deaths more than doubled from 2010 to 2015, but starting in 2013, more drug overdose deaths were coming from heroin and fentanyl. By 2015, prescription opioids accounted for only about 15 percent of all drug overdose deaths in the county.

Other states are seeing a similar trend. A review of drug overdose deaths in Massachusetts from 2013 to 2014 found that heroin or fentanyl was involved in 85 percent of all opioid-related deaths, with prescription opioids involved in only 22 percent.

In Cleveland, fentanyl’s toll has been even more dramatic, causing 86 percent of Cleveland’s reported drug overdose deaths as of August 2016. Commonly prescribed opioids were only found in 17 percent of overdose deaths.

CDC data reflects the trend at the national level. In 2010 and 2011, oxycodone (a commonly prescribed opioid) was the leading cause of drug overdose deaths nationally, but in 2012 heroin was responsible for more overdoses than oxycodone.

By 2014, deaths linked to heroin had more than tripled from 2010 levels, making heroin responsible for almost a quarter of all drug overdose deaths. And drug overdose deaths involving fentanyl more than doubled in just one year.

Health officials worry that drug overdose deaths from heroin could continue to climb as heroin use becomes more widespread. A 2017 JAMA study found that heroin use is not always linked to prescription opioid use, and part of its increased prevalence may be driven by commonly prescribed drugs like oxycodone obtained on the black market.

“Heroin use appears to have become more socially acceptable among suburban and rural white individuals, perhaps because its effects seem so similar to those of widely available prescription opioids,” wrote Silvia Martins and Columbia University researchers.