Heroin users are attracted to the drug not only for the “high” it gives them, but also because it’s cheaper and easier to obtain as compared to prescription drugs.

A survey conducted by researchers at Washington University School of Medicine in St. Louis looked at data collected from over 150 drug treatment centers across the U.S. Approximately 9,000 patients dependent on narcotics were surveyed, with nearly 2,800 admitting that heroin was their drug of choice.

Three key factors were noted in the survey: “High” enjoyment, ease of administration (through snorting or injecting), and accessibility.

Lead investigator Theodore J. Cicero, PhD and colleagues discovered that many heroin users started using the drug only after using prescription painkillers. In the past, the situation was switched, where heroin was often the “gateway” drug that introduced users to narcotics. The researchers also discovered that heroin users nowadays are mainly white men and women in their late 20s residing outside large urban areas.

“What we’re seeing now is that most people using heroin begin with prescription painkillers such as OxyContin, Percocet or Vicodin, and only switch to heroin when their prescription drug habits get too expensive,” said Cicero in a statement.

OxyContin is one of the most abused prescription painkillers, but in 2010, it was reformulated in such a way that it became much more difficult to crush or dissolve. Such reformulation made the drug more difficult to snort or inject, leading many users to switch to other drugs, such as heroin.

Making certain drugs more expensive and more difficult to snort or inject won’t stop users from taking drugs at all. Instead, they’ll simply switch to something more accessible, which many have found with heroin.

The results of the survey are published in the journal JAMA Psychiatry.