U.S. drug overdose deaths, which have been concentrated in Appalachia and other rural areas for more than a dozen years, are back to being most common in big cities again, according to a government report issued Friday.

The report by the Centers for Disease Control and Prevention said the urban overdose death rate surpassed the rural rate in 2016 and 2017. Rates for last year and this year are not yet available. But experts, citing available data, say the urban rate is likely to stay higher in the near future.

The difference between the urban and rural counties was not large. In 2017, there were 22 overdose deaths per 100,000 people living in urban areas, compared with 20 per 100,000 in rural areas.

The nation is battling the deadliest drug overdose epidemic in U.S. history. About 68,000 Americans died of overdoses last year, according to preliminary CDC statistics reported last month.

Experts believe the epidemic has been playing out differently in different parts of the country, and they say it is best understood by comparing geographic regions — Appalachia and the Northeast, for example.

The new CDC report looked at urban and rural overdose death rates for the nation overall. The researchers found both rates have been rising, but the urban rate shot up more dramatically after 2015 to surpass the rural rate.

New York, Chicago and Baltimore all reported dramatic spikes in overdose deaths in the last few years, and they are not alone.

Diego Cuadros, a University of Cincinnati researcher, said the CDC findings are consistent with what he and his colleagues have seen in Ohio.

"Most of the hot spots are in the urban areas," he said.

The CDC found the urban rates are driven by deaths in men and deaths from heroin, fentanyl and cocaine.

That probably is due to a shift in the current overdose epidemic, said Dr. Daniel Ciccarone, a drug policy expert at the University of California, San Francisco.

The epidemic was initially driven by opioid pain pills, which were often as widely available in the country as in the city. But then many drug users shifted to heroin and then to fentanyl, and the illegal drug distribution system for heroin and fentanyl is more developed in cities, Ciccarone said.

Another possible explanation is increasing overdose deaths among blacks and Hispanics, including those concentrated in urban areas, he added.

"Early on, this was seen as an epidemic affecting whites more than other groups," he said. "Increasingly, deaths in urban areas are starting to look brown and black."

Women still die of overdoses at higher rates in rural areas, the CDC report found. And death rates tied to methamphetamine and prescription opioid painkillers remain higher in rural areas, too.

Using death certificate data, the CDC researchers looked at whether overdose victims were living in rural or urban counties at the time they died. They defined urban areas as counties with large and small cities and their suburbs. Rural areas were non-suburban counties with fewer than 50,000 residents.

The report looked at trends from 1999 through 2017. Overdose death rates for 2018 are to be reported later this year.

The urban and rural death rates were nearly identical for people ages 25 to 44 — the age group with the worst fatal overdose problem. "Drug epidemics tend to affect young people," Ciccarone said.

But the urban rate was significantly higher in other age groups, particularly in those ages 45 to 64. Experts interviewed by The Associated Press said it's not clear why the urban overdose death rates were markedly higher for middle-aged and older Americans.

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