THEY have America in a deadly grip. In 2015, the most recent year for which full statistics are available, 33,091 Americans died from opioid overdoses, according to the Centres for Disease Control—almost three times the number who perished in 2002. Nearly as many Americans were killed by opioids in 2015 as were killed by guns (36,132) or in car crashes (35,092). In the state of Maryland, which releases more timely figures, drug-overdose deaths were 62% higher in the first nine months of 2016 than a year earlier.

The opioid epidemic is quite unlike past drug plagues. Deaths are highest in the Midwest and north-east, among middle-aged men, and among whites. Some of the worst-affected counties are rural. In 2013 a 40-year-old woman walked into a chemist’s shop in the tiny settlement of Pineville, West Virginia, pulled out a gun, and demanded pills. Don Cook, a captain in the local sheriff’s department, says he continues to nab many people for illegally trading prescription painkillers.

The epidemic is, in short, concentrated in Donald Trump’s America. (Commendably, Mr Trump raised the danger of opioids on the campaign trail; sadly, he has done little since becoming president beyond setting up a commission.) It has even been argued that the opioid epidemic and the Trump vote in 2016 are branches of the same tree. Anne Case and Angus Deaton, both economists at Princeton University, roll opioid deaths together with alcohol poisonings and suicides into a measure they call “deaths of despair”. White working-class folk feel particular anguish, they explain, having suffered wrenching economic and social change.

As an explanation for the broad trend, that might be right. Looked at more closely, though, the terrifying rise in opioid deaths in the past few years seems to have less to do with white working-class despair and more to do with changing drug markets. Distinct criminal networks and local drug cultures largely explain why some parts of America are suffering more than others.

Opioids can be divided into three broad groups. First, and most notorious, are legitimate painkillers such as OxyContin. Heavily prescribed from the 1990s, some of these pills were abused by people who defeated their slow-release mechanisms by crushing and then snorting or injecting them. The second group consists of powerful synthetic opioids such as fentanyl and carfentanil. These have legitimate medical uses, but are often manufactured illicitly and smuggled into America. The third opioid is heroin, derived from opium poppies, almost all of it illegally.

Until about 2010 the rise in opioid deaths was driven by the abuse of legitimate painkillers, which are sometimes called “semi-synthetic” because they are derived from plants. In the past few years, though, heroin and synthetic opioids have become bigger threats (see chart 1). Some addicts have moved from one class of opioid to another. The Drug Enforcement Administration (DEA) estimates that almost four out of five new heroin users previously abused prescription drugs.

OxyContin pills can no longer be crushed as easily, and doctors have become more wary of prescribing powerful painkillers. As a result, between 2012 and 2016 opioid prescriptions fell by 12%. Heroin can be cheaper and easier to obtain. According to one narcotics officer in New Hampshire, a 30-milligram prescription pain pill sells for $30 on the street. A whole gram of heroin can be had for $60-80.

Fentanyl is cheaper still. It is often made in Chinese laboratories and smuggled into America; some traffickers obtain it through the dark web, an obscure corner of the internet. Fentanyl is usually added to heroin to make it more potent or is made into pills, which can resemble prescription painkillers. Because it is such a powerful drug—at least 50 times stronger than heroin—the smuggling is easy and the potential profits are huge. One DEA official has explained that a kilogram of fentanyl from China costs about $3,000-5,000 and can be stretched into $1.5m in revenue in America. By comparison, a kilogram of heroin purchased for $6,000 translates to $80,000 on the street.

Yet not all addicts make the switch from one kind of opioid to another. In West Virginia, Mr Cook hardly ever encounters heroin—perhaps, he suggests, because no major highway runs through his patch. Whereas the death rate from prescription painkillers is more or less the same in America’s four regions, deaths from heroin and synthetic opioids are high in the Midwest and north-east, middling in the South and low in the West (see chart 2). All eight states where police agencies reported 500 or more encounters with fentanyl in 2015 are east of the Mississippi river.

“Once a drug gets into a population, it’s very hard to get it out,” explains Peter Reuter, a drugs specialist at the University of Maryland. “But if it doesn’t get started, it doesn’t get started.” It is never entirely clear why a drug catches on in one place but not another. There is, however, a possible explanation for why heroin and synthetic opioids have not yet taken off in western states: the heroin market is different.

Although most heroin enters America from Mexico, there are really two trafficking routes. Addicts west of the Mississippi mostly use Mexican brown-powder or black-tar heroin, which is sticky and viscous, whereas eastern users favour Colombian white-powder heroin. According to the DEA, in 2014 over 90% of samples classified as South American heroin were seized east of the Mississippi, while 97% of Mexican heroin was purchased to the west. The line is blurring—Mexicans are pushing into the white-powder trade, and black tar is creeping east—but it still exists.

White-powder heroin looks much like a crushed pain pill, making it comparatively easy to switch from one to the other. It is also fairly easy to mix white-powder heroin with a powder such as fentanyl. Black tar is more distinct and harder to lace with other substances because of its stickiness and colour; mixing in white powder can put buyers off. “The lore on the street is: the lighter in colour brown-powder or black-tar heroin is, the less heroin it has,” says Jane Maxwell, a researcher at the University of Texas at Austin.

The West’s distinctive heroin market has probably deterred many painkiller addicts from trying the drug, and has kept synthetic opioids at bay. Outbreaks have occurred, though. In just two weeks in 2016, 52 people overdosed and 14 ultimately died near Sacramento, in California, after taking counterfeit hydrocodone pills laced with fentanyl. In New Mexico, fentanyl disguised as black-market oxycodone is thought to have killed 20 people last year. This is a rare case where one should pray that America stays divided.