Deaths caused by heroin use have rapidly increased since 2010 as those who abuse prescription painkillers have turned to it instead of more strictly regulated medical opioids such as oxycodone.

According to a study from the Centers for Disease Control and Prevention, heroin-related deaths nearly tripled from 2010 to 2013 in the United States, from 3,094 to 8,257. The number of heroin users nearly doubled from 2007 to 2013, from 373,000 to 681,000.

During the same period, however, deaths caused by prescription painkillers began to decrease.

The shift is the result of many factors. Heroin is now cheaper and easier to access than prescription painkillers, which are used by record numbers across the country.

Time reported: “This rise in cheaper, purer and more readily available heroin has coincided with a law enforcement crack down on illegal prescription pill providers. Some 6.8 million Americans abuse prescription pills, according to SAMHSA’s [Substance Abuse and Mental Health Services Administration] 2012 National Survey on Drug Use and Health, and popular opiates like OxyContin, Percocet, and Vicodin have become increasingly hard and costly to obtain on the black market in recent years. In 2010, OxyContin changed its formula to make the pill harder to crush and dissolve, and, therefore, abuse.”

Since heroin and many prescription painkillers derive from opium, they have similar psychological effects. A SAMHSA study in 2012 revealed that individuals who abused painkillers were 19 times more likely to try heroin than those who had not.

In response to greater demand, Latin American cartels have begun distributing higher-quality heroin deeper into the U.S., especially in Midwestern states. This has made it more palatable for those who would not normally use it.

“When heroin was cut with so much filler, it required a user to inject it into their body to achieve the desired high,” Time reported. “The purer versions currently available can be smoked or snorted, which make them more appealing to teenagers, the college-educated and ‘people who normally wouldn’t come near it for fear of the needle,’ says [Jack] Riley [a special agent in charge of the Drug Enforcement Administration’s Chicago Field Division]. ‘That’s why it is spreading.’”

Officials also attribute the increased number of heroin deaths to the fact that the illicit substance is being manufactured differently. The DEA correlated the greater number of heroin-related deaths to the presence of potent additives such as fentanyl, which augments the effects of heroin up to 50 times.

“[In 2014], law enforcement [officials across the United States] found 3,344 samples of drugs containing fentanyl, more than triple the total in 2013,” The Wall Street Journal reported.

“At a Senate hearing…[Administrator of the DEA] Ms. [Michele] Leonhart pointed to fentanyl as a key driver of a doubling in the number of heroin-related deaths in Maryland last year.

“‘We’re looking at those deaths, and we’re finding that a number of them are actually fentanyl-laced heroin overdoses,’ she said.

“Hundreds of deaths in Pennsylvania over the past two years have also been linked to fentanyl, with users in some cases injecting pure forms of the drug.”

In addition, some heroin buyers used to abusing the less potent product more easily overdose, which can lead to death.

Although post-2013 national figures have not yet been compiled, the heroin death rate appears to show no sign of slowing down.