The three Vancouver users have become the first in North America to be given free prescription heroin, as part of a controversial experiment.
The North American Opiate Medication Initiative, known as NAOMI, will eventually recruit 157 people for the two-year study.
"Today, the treatment stage of the NAOMI study begins," said spokesman Jim Boothroyd. "The clinical trial is fully up and running."
Participants will be split into two groups — one receiving heroin, the other methadone — to find out if heroin is better for addicts who have failed at methadone or abstinence.
The heroin users will attend NAOMI'sslum three times a day, seven days a week. They are given their fix and shoot it themselves in an on-site safe-injection room under nurse supervision.
"The site has the security requirements of Fort Knox," said Boothroyd. "There's very little heroin on the site, and the site is extremely secure."
Methadone users will come twice a day to drink their heroin substitute.
The participants get free drugs and medical care, but will only make $150 over the trial by filling out a half-dozen questionnaires.
The synthetic heroin is made in Europe, and is stored in a secret location.
More than 4,000 drug addicts live near the clinic in Vancouver's impoverished Downtown Eastside, an area known for its street drug deals.
The 157 people selected to take part must have been addicted for at least five years.
Those in the project will go to the clinic three times a day. Nurses will supervise the injections. Seventy people will receive methadone; the rest will get heroin and methadone.
NAOMI's clinical head, Dr. David Marsh, said participants will get a measured dose of heroin that will be reduced if they top up with street drugs.
"This will be the first time in Canadian history that physicians are prescribing heroin for the treatment of heroin dependence," he said.
Kim Kerr, executive director of the Downtown Eastside Residents Association, said it's time to stop complaining about addiction and start treating it.
"People who are addicted to heroin should be treated like anyone with a medical condition," he said.
By John Bermingham