Concern over the U.S. drug epidemic has focused on the growing abuse of prescription painkillers like oxycodone. But if overdose deaths are any indication, heroin may still be a bigger problem.

Death rates from heroin overdoses have nearly quadrupled since 2000 to reach 2.7 per 100,000 people in 2013, according to a report released Wednesday by the National Center for Health Statistics, part of the Centers for Disease Control and Prevention.

And much of that growth occurred after 2010 — 37 percent each year from 2010 to 2013, the report said, from 1 death per 100,000 people to 2.7 per 100,000 people. Death rates from nonopioid painkillers declined slightly after 2010, from 5.4 deaths per 100,000 people to 5.1 per 100,000 in 2013.

"Identifying populations at high risk of heroin-related drug-poisoning death can help target prevention strategies," the report said.

Heroin overdoses by race are also shifting. In 2000 non-Hispanic blacks age 45 to 64 had the highest heroin overdose death rate, at two deaths per 100,000 people. But in 2013, non-Hispanic whites age 18 to 44 vastly outpaced them, with 7 deaths per 100,000 people.

In terms of gender, men die from heroin overdoses four times as often as women, according to the report. Male death rates quadrupled to reach 4.2 per 100,000 people in 2013, from 1.6 per 100,000 people in 2010. But female overdose deaths are also on the rise, tripling to reach 1.2 per 100,000 people in from 0.4 per 100,000 people from 2000 to 2013.

Heroin overdose death rates spiked across the United States. But they rose most rapidly in the Midwest, the report said.

While the changing demographics of heroin overdose deaths surprised researchers, they said the findings would have to be combined with other data to explain why heroin overdose deaths are on the rise.

“Unfortunately we don’t know the reason why,” said Dr. Holly Hedegaard, an injury epidemiologist at the National Center for Health Statistics and a co-author of the report. “That’s the big question.”

One obvious clue is found in the expanded use of heroin in recent years. The number of current heroin users — meaning that they have used the drug within the last month — in the U.S. has more than doubled in the last decade, from 119,000 in 2003 to 289,000 in 2013, according to the National Survey on Drug Use and Health (PDF), which is released by the Department of Health and Human Services.

Sharon Kelley, the chief executive officer of the Associates in Emergency Medical Education, offers another possible explanation. She says that law enforcement’s crackdown on prescription drug abuse has led addicts to turn to hospital emergency departments, or to heroin.

“What has happened is you see a reduced availability on the street,” said Kelley, a former police officer who has a Ph.D. in toxicology. “Patients will come in and tell the doctor, ‘I want this drug,’” she said, adding that hospital emergency departments are seeing increasing amounts of patients trying to score drugs when their doctors won’t comply.

“The majority of people who get hooked on opioids are just normal people who start on pain medication,” Kelley added.

Last August the DEA tightened restrictions on narcotic painkillers such as Vicodin that are made with hydrocodone, reclassifying them as Schedule II drugs under the federal Controlled Substances Act to try to curb abuse and addiction. That places them in the same category as codeine and oxycodone. The new restrictions also allow just one 90-day prescription of the drugs at a time and bar doctors from giving refills over the phone.

Now that heroin is widely available in inhalable or smokeable form, Kelly said, it seems less frightening. "Now they don’t have to shoot it anymore," she said.

David G. Evans, of the Drug-Free America Foundation, blames the use of less-harmful drugs for the uptick in heroin use.

“I think heroin deaths are going up in part because the attitude has changed towards marijuana,” he said. “Kids now think marijuana is a medicine so it’s okay to use it. You get involved in a drug culture, you get high, you get addicted, one drug just seems like the other, and I think that’s a big part of it.”

Both Evans and Kelley cited the need for greater availability and training for how to use naloxone, an FDA-approved antidote that can be administered during a drug overdose. It is being used by an increasing number of first responders such as police and fire departments. But Kelley cautioned that it wasn’t a solution to the many underlying problems that cause heroin use.

“We need to get people off of opioids and actually start treating the root cause again, because these drugs are so potentially addictive. If we can start treating the actual cause rather than the symptoms, I think we’d be a lot better off,” Kelley said. “Because it’s just too easy to give everybody a pill.”