Heroin use in the US has more than doubled since 2007. In 2013, more than 500,000 people said that they had used heroin in the past year — a nearly 150 percent increase in just six years, according to a CDC report released today. And what's worse is that almost all people who use heroin also use at least one other drug. Taken together, these two findings might explain why heroin claimed almost four times as many lives in 2013 as it did in 2000.

Heroin use is "increasing rapidly across nearly all demographics," says CDC director Tom Frieden. "And with that increase we're seeing a dramatic rise in deaths."

Heroin-related deaths nearly tripled between 2010 and 2013 — rising to 2.7 deaths for every 100,000 people, from one death for every 100,000 people. But beyond the increase in deaths, the rise in injection drug use is a serious health concern. Health officials have reported clusters of hepatitis C and HIV coming from opioid injection, Frieden says. "It's heartbreaking to see the return of injection drug use in the US."

The rise in heroin use has a lot to do with with the painkiller addiction and the low cost of heroin, the CDC says. So although more people die of painkiller related overdoses in the US, heroin's rising popularity is pretty worrisome — so worrisome that the CDC is calling the trend an "epidemic."

Today, those who are most at-risk for heroin dependence are non-Hispanic whites, people between the ages of 18 and 25, Medicaid recipients, men, people who don't have health insurance, and people who make less than $20,000 a year. "The greatest increases are among populations with high use of prescriptions opioids," says Christopher Jones, senior advisor at the Office of the Commissioner at the FDA.

"Heroin users are no longer inner-city minority groups but now are more likely to be younger whites living outside the large urban area," Jane Carlisle Maxwell, a political scientist at the University of Texas at Austin, wrote in a 2015 study published in the Journal of Addictive Diseases. In 2013, white people ages 18 to 44 had the highest rate of heroin-related deaths of any group — seven for every 100,000 people. By way of comparison, two deaths for every 100,000 people occurred among the highest-risk group in 2000 — and that group was non-Hispanic blacks ages 45 to 64.

Some researchers have linked the rise in heroin use to measures aimed at decreasing the non-medical use of prescription drugs: changing Oxycontin's formula to make it harder to crush and snort, and a general decrease in doctors' willingness to prescribe addictive pain-relievers. To avoid withdrawal, people who were used to taking pills turned to heroin, suggests Carlisle Maxwell, who previously served on the Substance Abuse and Mental Health Services National Advisory Council. But Frieden disputes this idea.

"Improving prescribing practices for prescription opiates is part of the solution, not part of the problem," Frieden says. He suggests that an increase in the supply of heroin and a decrease in the drug's cost — "the cost of heroin is roughly five times less than prescription opioids on the streets," Frieden says — are the main contributing factors in heroin's rising popularity. That, combined that with the fact that people who use painkillers are already "primed" for heroin addiction, might explain the rise in heroin use, he says. That's why he thinks reducing the over-prescription of painkillers can help reduce people's tendency to turn to heroin in the first place.

The "shift toward heroin started long before states started addressing [painkillers]," Jones agrees. People who use painkillers "will start to use heroin, and then shift back to opioids depending on what's available."

Regardless, one thing is clear: the strongest risk factor for heroin use is painkiller addiction. "People who abuse or who are dependent on prescription painkillers were 40 times more likely to abuse heroin," Frieden says.

Previous studies have found that about 80 percent of current heroin users report having used prescription painkillers in the past, but only one percent of people who use painkillers report previous heroin use. And almost all people who use heroin also use at least one other drug — with 61 percent reporting the use of at least three different drugs.

Using multiple drugs probably contributes to the high rate of heroin-related deaths. In 2013, 59 percent of the 8,257 heroin-related deaths seen across the country involved at least one other drug.

Overall, the CDC's report is pretty dispiriting. But reversing these numbers is still possible, Frieden says. Curbing doctors' tendencies to over-prescribe painkillers should reduce the use of heroin, Frieden suggests. And states can help save lives by increasing the availability of treatment facilities and safe injection equipment, and by ensuring that more people are trained to administer life-saving drugs like Naloxone during an overdose.

"Changes at the state levels are particularly important," Frieden says. "We can turn it around by a package of public health care and law enforcement activities."