A study of people addicted to painkillers who began using heroin confirms that many of them were drawn to heroin’s low cost and wide availability.

Researchers surveyed 15,000 people being treated for drug addiction, and conducted in-depth interviews with 267 of them. They found almost half of those interviewed were addicted to opioid painkillers such as OxyContin, Vicodin and Percocet before using heroin, HealthDay reports.

Three-fourths of those interviewed said they tried heroin because of its lower cost and greater availability.

Lead researcher Theodore Cicero of Washington University in St. Louis said heroin use began rising after the federal government began shutting down “pill mills” and doctors who illegally prescribe opioids.

“Prescription narcotics have become so expensive, and the access is fairly limited, that a number of people who would have previously shunned using heroin are now willing to use it because it is more accessible and cheaper,” said Cicero.

The study, published in the New England Journal of Medicine, was conducted between 2008 and 2014. The researchers found exclusive use of painkillers dropped from 70 percent to less than 50 percent during that period. Concurrent use of heroin and painkillers rose from 24 percent to 42 percent. Exclusive use of heroin rose from 4 percent to 9 percent.

“We see very few people transition completely from prescription opioids to heroin; rather, they use both drugs,” Dr. Cicero said in a news release. “There’s not a total transition to heroin, I think, because of concerns about becoming a stereotypical drug addict.” He added, “If users can’t get a prescription drug, they might take whatever else is there, and if that’s heroin, they use heroin.”