Many people who abused the painkiller OxyContin by inhaling or injecting it switched to heroin after the prescription's formula changed, U.S. researchers say.

OxyContin was designed to slowly release the opioid drug oxycodone. After people started abusing it by crushing the pills and inhaling the powder or dissolving the pills in water and injecting it to get a rush, the drug maker introduced a new formula in the U.S. in 2010 to make inhaling and injecting more difficult.

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An abuse-deterrent formulation of OxyContin may not be a "magic bullet" for solving opioid abuse problems. (AP)

The new pills are harder to crush and dissolve more slowly.

In Wednesday's issue of the New England Journal of Medicine, investigators in the U.S. said use of OxyContin by inhalation and injection has dropped significantly since the abuse-deterrent form went on the market.

"In that sense, the new formulation was very successful," said author and principal investigator Theodore Cicero, a professor of neuropharmacology in psychiatry at Washington University School of Medicine in St. Louis, Mo.

"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," he added in a release.

OxyContin was a popular drug in suburban and urban areas, where drug abusers have now shifted either to more potent opioids if they can find them or to heroin, he said.

Cicero's team collected survey information from more than 2,500 patients entering treatment for drug abuse in 39 states starting in 2009.

Cheaper way to get high

Nearly 36 per cent of abusers said OxyContin was their main drug before the new formulation came out, which dropped to about 13 per cent about 21 months later, the researchers said.

At the same time, selection of high-potency Fentanyl and hydromorphone rose "markedly" from 20 per cent to 32 per cent.

Earlier this week, police in Sarnia, Ont., said Fentanyl is becoming a killer street drug.

The U.S. researchers asked abusers why they had switched.

"These changes appear to be causally linked, as typified by one response: 'Most people that I know don't use OxyContin to get high anymore. They have moved on to heroin [because] it is easier to use, much cheaper and easily available," the researchers wrote in a letter to the journal.

They concluded that abuse-deterrent formulations may not be the "magic bullets" many hoped for in solving the opioid abuse problems.