Injections of the active ingredient of heroin work far better than oral methadone for keeping addicts in treatment, away from illegal drugs and out of trouble, Canadian researchers reported on Wednesday.

But the researchers cautioned that the treatment carries a risk of overdose and seizures, so the injections should only be done in a medical setting.

"Methadone, provided according to best-practice guidelines, should remain the treatment of choice for the majority of patients," Eugenia Oviedo-Joekes of the University of British Columbia and colleagues wrote in the New England Journal of Medicine.

But when that does not work, they said, giving heroin's active ingredient, diacetylmorphine, appears to be an effective alternative. Methadone treatment fails in 15 to 25 percent who seek care.

Oviedo-Joekes and colleagues studied 226 addicts in Montreal and Vancouver. Only 54 percent of those who got methadone stayed in treatment for a year, they found.

But 88 percent of those who got diacetylmorphine shots did. And those who got diacetylmorphine cut back on illicit drugs by 67 percent, compared to 48 percent who got methadone.

All the addicts who received diacetylmorphine spent at least 45 minutes per visit in the clinic to check for side effects. Some were there three times a day.

While three quarters of the addicts said they had engaged in non-drug-related illegal activities in the previous month, the rate dropped to 5.4 percent with methadone treatment and 0.9 percent for the diacetylmorphine recipients.

"Once you open your mind to the idea of getting people off the streets, out of crime, out of the back alleys and into clinics where they're going to be treated by doctors and nurses and counselors, some people will say, 'Ah ha! I get it now.' They begin to see the pragmatic side of the argument," Dr. Martin Schechter of the University of British Columbia, who worked on the study, said in a telephone interview.

The cost of heroin treatment is less than $10,000 per year. "We know a person who is out of treatment, their cost to society is over $50,000 a year, and that includes emergency room visits, doctors, courts, police and jail and so on," he said.

North America has about a million heroin addicts.

The results are expected to have little impact in Europe, where similar tests have produced comparable results.

Switzerland has been prescribing heroin for a decade.

"The prescription of heroin is now recognized in some European countries as the optimal treatment for patients for whom options are running out and in whom methadone maintenance has not worked, and it keeps the user in contact with drug services," Virginia Berridge of the University of London wrote in a commentary.