WASHINGTON — Ever since the popular painkiller pill OxyContin became harder to crush into powder two years ago, many US drug abusers have turned to heroin instead, researchers said Wednesday.

The formula change did not appear to stop people from snorting or shooting up, but rather caused them to switch to a dangerous street drug that mimics the same high, said a letter to the editor in the New England Journal of Medicine.

Highly addictive OxyContin shot to popularity in the last decade, largely in rural parts of the eastern United States, gaining the nickname "Hillbilly Heroin," and causing its producers to devise a new pill that would not disintegrate when pounded in a bid to stem the epidemic of abuse.

"The most unexpected, and probably detrimental, effect of the abuse-deterrent formulation was that it contributed to a huge surge in the use of heroin, which is like OxyContin in that it also is inhaled or injected," said Theodore Cicero of the Washington University School of Medicine.

"We're now seeing reports from across the country of large quantities of heroin appearing in suburbs and rural areas."

Cicero and colleagues are analyzing survey data collected from 2009-2012 and completed by 2,566 patients who were entering treatment programs for prescription opioid dependence at various locations in the United States.

At the beginning of the research period, 35.6 percent of patients said OxyContin was their primary drug of choice, a number which has since fallen to 12.8 percent, the researcher said.

When asked to identify any opioids used in the last 30 days to get high at least once, OxyContin was mentioned by nearly half at the start, but fell to 30 percent by 2012.

Meanwhile, heroin use almost doubled from around 10 to nearly 20 percent.

Further interviews with about 100 of the survey participants showed that a common reason for the switch to heroin was the decreased availability of crushable OxyContin.

"This trend toward increases in heroin use is important enough that we want to get the word out to physicians, regulatory officials and the public, so they can be aware of what's happening," said Cicero, who is continuing to analyze the data.

"Heroin is a very dangerous drug, and dealers always 'cut' the drug with something, with the result that some users will overdose. As users switch to heroin, overdoses may become more common."

His letter concluded that "abuse-deterrent formulations may not be the 'magic bullets' that many hoped they would be in solving the growing problem of opioid abuse."

The US Centers for Disease Control and Prevention said last year that about 12 million Americans report taking prescription painkillers for recreational uses, though the actual figure is suspected to be much higher.

Opioid pain relievers, including oxycodone (OxyContin), methadone and hydrocodone, better known as Vicodin, have quadrupled in sales to pharmacies, hospitals and doctors' offices since 1999.

The number of deaths from overdoses of opioid pain relievers has more than tripled from 4,000 people in 1999 to 14,800 people in 2008, and now surpasses the number of deaths from cocaine and heroin combined, the CDC said.