The opioid epidemic in the US can sometimes seem like a giant game of whack-a-mole for health officials — look away and the epidemic shifts. More people still overdose on prescription painkillers than any other opioid, but heroin and other illicit opioids like fentanyl are now fueling a separate, and perhaps even deadlier, drug epidemic.

And it turns out there are now big differences between age groups when it comes to opioid overdoses. Jay Unick, a professor at the University of Maryland and specialist in the health consequences of heroin use, found a stark divide in his analysis of drug overdose and emergency room data from 2013 and 2014: Americans in their 50s and 60s overwhelmingly overdosed on prescription opioids, while Americans in their 20s and 30s overdosed disproportionately on heroin.

Researchers first observed a shift in how people between the ages of 20 and 35 overdosed on drugs in 2007. The dramatic uptick in the use of heroin has continued since then, claiming more than 12,000 lives in 2015.

Unick, who presented his findings last week at the National RX Drug Abuse and Heroin Summit, thinks the age divide is an unintended consequence of states moving to crack down on opioid prescriptions. In other words, restrictions on prescription painkillers are driving younger Americans to use heroin and other opioids on the black market.

“Older people have greater access to pills,” said Unick — and many still have a pathway to obtain opioids legally because of either chronic pain or disability. “But younger people are just starting with heroin and aren’t even making that shift from pills to heroin.” (Heroin-related deaths are increasing for older Americans too — just not nearly as fast.)

Unick is currently working on a paper to publish his findings from the Agency for Healthcare Research and Quality (AHRQ) hospitalization data and Centers for Disease Control and Prevention data he analyzed. But at this stage they are preliminary and haven’t been peer-reviewed. You can access the presentation slides he shared with Vox here.

Heroin use varies dramatically in the US. But the problem is most serious in the Northeast and Midwest.

When we talk about the opioid epidemic in the US, much of the conversation centers on West Virginia, Kentucky, and rural Appalachia, where communities have been especially hard hit. In West Virginia, for instance, the drug overdose death rate is nearly three times the national average at 41.5 deaths for every 100,000 people.

But in his analysis of hospitalizations and emergency room visits, Unick found something unexpected — hot spots for heroin abuse were largely concentrated in the Northeast and Midwest. (He used hospitalization and emergency room data to better capture the magnitude of the problem, as drug overdose data only includes people who died.)

As you can see in the chart below, though, that is changing, as regions (excluding the West) have experienced a steady increase in emergency room visits from heroin overdoses since 2011.

To be clear, more people have died from prescription opioids than from heroin. But — and this is key — there isn’t as much regional variation in prescription overdose deaths as there is with heroin deaths. What’s more, as the chart below shows, hospitalizations from prescription painkillers started to decline in all regions of the US as of 2012.

It’s just another piece of evidence that the opioid epidemic is changing. And other data shows that increasingly, heroin and fentanyl are emerging as deadly drivers of the epidemic.

For the first time in 2015, heroin killed more people than prescription painkillers (even though a significant number of people still died from prescription opioids).

One thing that is apparent is the recent increase of fentanyl and heroin use has already devastated parts of the US, and states like Ohio are in the grips of a full-blown heroin crisis. As you can see in the chart below, Ohio has double the heroin overdose rate of its neighboring states.

And Unick fears states’ response of clamping down on pills won’t be enough to stem the crisis. “We’re going to get stuck if we don’t increase access to treatment,” he said. “A doctor can prescribe you hydrocodone but can’t prescribe [treatment] drugs like naloxone because of stigma. We can prescribe the drugs that kill you but not the drugs that save you.”

Studies have found naloxone to be effective in reducing the risk of opioid overdose. But as Vox’s German Lopez reported, only 45 states (plus Washington, DC) have laws that increase access to naloxone — and even then, the level of protection and access under the law can vary dramatically from state to state.