The Massachusetts heroin epidemic is unlike any other in the United States. The overdose rate in the state is more than twice the national average. And deaths from prescription opioids like OxyContin are only slightly less harrowing.

Unusual, too, is the degree to which these two scourges are feeding off each other. A substantial and spiking number of overdoses in Massachusetts involves both heroin and prescription drugs, something you rarely find elsewhere in the United States.

Until now, it’s been hard to see how, exactly, heroin and prescription opioids were interacting, since almost all available data lump them together under the heading of “opioids.’’ But a Globe examination of the information in death certificates from 1999 to 2014 reveals the increasingly toxic interplay between the drugs, both at the state level and in individual counties.

Here’s what those death certificates tell us:

¦ Half of all opioid deaths in Massachusetts in 2014 involved heroin. This is a very new development. As recently as 2010, heroin was implicated in less than 20 percent of opioid deaths.

¦ Prescription opioid overdoses seem to be soaring again after a brief plateau. Between 2013 and 2014, they increased by over 90 percent.

¦ Massachusetts isn’t like other states. Elsewhere in the country, heroin and prescription opioids tend to kill people of different ages — heroin strikes the young, prescription drugs the middle-aged. Not so in Massachusetts, where 25- to 34-year-olds increasingly bear the brunt of both.

¦ County by county, there are some big variations. In the Berkshires, and outside the big cities, heroin is the dominant killer. Across the southeast portion of the state, however, it hasn’t made the same inroads.

Heroin vs. prescription drugs

Initially, the big problem was prescription opioids like OxyContin and Vicadin. Between 1999 and 2010, Massachusetts saw a six-fold increase in deaths from such prescription opioids — a rapid change, to be sure, but consistent with what other states were experiencing.

That changed in 2010, when heroin was added to the mix and Massachusetts diverged from the national pattern. Heroin deaths across the Bay State rose faster than prescription drug overdoses ever did, far outpacing national numbers.

Ominously for Massachusetts, the rise of heroin seems to have reenergized demand for prescription opioids. Two features of this return to prescription opioids make it especially concerning.

First, a growing number of users seem to be overdosing on a combination of heroin and prescription opioids. That’s both new and rare. In 2013, death certificates record only 36 such examples. In 2014, that jumped to 179, easily among the highest levels in the country. One possible culprit is Fentanyl, a prescription opioid increasingly being used as a heroin additive, boosting potency and virulence (another explanation would be a regulatory shift affecting death certificates, but the Massachusetts Department of Public Health said it was unclear whether that was a factor.)

Second, the victims are getting younger. In the past — and all across the country — prescription opioids tended to kill older people, particularly those ages 45 to 54. But in 2014, the bulk of prescription opioid deaths in Massachusetts involved people ages 25 to 34 — the very same age group disproportionately affected by heroin.

Put these together and it really does seem that the heroin and prescription opioid epidemics are joined in Massachusetts in a way that they simply aren’t elsewhere. And that their combination is helping to drive the state’s surging opioid crisis.

County by county

The mix of heroin and prescription opioids varies quite a bit within the state.

Heroin deaths are concentrated in the central and eastern part of the state — and nowhere are they higher than in northeast Essex County.

But the shift from prescription drugs to heroin is happening all over. In the areas around Boston, Worcester, and Springfield — and again in Berkshire County to the west — heroin now plays a role in well over 50 percent of opioid-related deaths. In Hampden County, surrounding Springfield, it’s over 70 percent, a sign of the deep penetration of heroin in the Interstate 91 corridor.

Only Bristol County, in the southeast, has resisted the incursion.

There, prescription opioids remain the dominant killer, though it’s not clear whether that’s because this area is ­somehow less susceptible to heroin, or if it’s merely a matter of time.

The state and its neighbors

Only four states have a higher heroin death rate than Massachusetts, and two of those states are here in New England. In fact, Connecticut, New Hamphsire, Rhode Island, and Vermont all join Massachusetts as five of the 10 worst-hit states in the nation.

There are a couple of possible explanations for why heroin has become endemic to New England. It could be cultural transmission; users share their experience and expertise with neighbors, who pick up the habit and share with their own friends and peers.

However, the New England addiction also reflects the geography of drug smuggling and dealing. Once heroin traffickers establish an effective trade route, they have a strong incentive to expand their local business, leveraging their networks to move more drugs.

Most heroin comes to New England from Mexico, stopping for processing in New York before traveling up major highways, including I-91 and Interstate 95. From there, every exit becomes an untapped market, a way for dealers to reach new users and spread the disease.

What the future holds

As revealing as the death certificate data are, the information only goes through 2014, which leaves us 16 months behind a fast-evolving epidemic. And every few weeks seems to reveal a cluster of new opioid-related deaths, including recent reports of over 20 deaths in Middlesex County.

Since taking office in 2015, Governor Charlie Baker has pressed for greater state action, spearheading initiatives to improve treatment, to increase the use of overdose-prevention drugs like Narcan, and to limit the availability of prescription opioids.

It’s unclear, however, whether these efforts can curb the accelerating appeal of heroin or stem the tide of opioid deaths.

One fundamental challenge is that we don’t know what’s causing the problem in the first place.

Economic hardship doesn’t seem to be the driving factor, since the rise of heroin has accelerated even amid a growing economy. And while it’s tempting to think in terms of a chain of addiction — the rise of prescription opioids drew users, who then switched to heroin — this too seems inadequate. Many other states saw a similar rise in prescription opioids, without a subsequent heroin spike.

Absent a single solution, the multipronged approach seems to make the most sense, including a combination of heightened anti-trafficking efforts, better treatment, and ongoing public education.

And given that opioids have become a marked problem for our region as a whole, state policy alone may not suffice. This Massachusetts crisis may require a New England solution.

Evan Horowitz digs through data to find information that illuminates the policy issues facing Massachusetts and the United States. He can be reached at evan.horowitz@globe.com. Follow him on Twitter @GlobeHorowitz