Briefly inhaling cannabis three times a day eases a kind of chronic pain that affects tens of thousands of Canadians — without making them high — Montreal researchers are reporting.

The new study, the first clinical trial in the world to allow patients to take marijuana home with them and "self-dose," found that for people with neuropathic pain — a common and dreaded condition that causes electric, stabbing pain — smoking cannabis reduced pain, improved mood and helped them sleep.

Three different potencies (2.5 per cent, six per cent and 9.4 per cent) of THC, the active ingredient in marijuana, were tested against a placebo, or "dummy" pot in 21 patients with neuropathic pain, none of whom had responded to standard treatments.

Participants inhaled a single dose through a pipe three times daily for five days, followed by a nine-day "washout" period. They were instructed to inhale for five seconds while the cannabis was lit, hold the smoke in their lungs for ten seconds, and then exhale.

Pain was measured on an 11- point scale ranging from "no pain" to "worst pain possible."

Patients reported less pain, better sleep and less anxiety when they were smoking the highest concentration of THC, compared with the placebo.

It wasn't a massive reduction in pain: The average daily pain intensity was 5.4 with 9.4 per cent THC, versus 6.1 with the placebo.

"But the patients that we were recruiting had to be patients that had tried and failed all other conventional treatments," said lead author Dr. Mark Ware, director of clinical research at the Alan Edwards Pain Management Unit at the McGill University Health Centre.

Neuropathic pain — which is caused by damage to nerves — affects as much as two per cent of the population. The Montreal study involved patients with post-trauma or post-surgical pain — radiating pain from prolapsed discs, for example, or burning pain from an incision.

"Through no fault of the surgeon's, it's impossible to do an operation without cutting through some nerves," Ware said. "For some people, for some reason, even after the tissue has healed they have persistent pain at the site of the operation.

"All of these were very refractory, very difficult pain problems to resolve. Any improvement in pain was quite remarkable."

As many as 15 per cent of patients with chronic non-cancer pain and multiple sclerosis report using marijuana, and Ware said that when he asked his own pain patients, similar numbers reported using marijuana. "There clearly was an unmet need."

"We're not going to argue that smoked cannabis is the be-all and end-all of pain management," Ware said.

"It simply opens the door to the fact that cannabinoids may be an additional tool in the physician's tool box, along with other medications and other non-pharmacological approaches."

During the study, "psychoactive effects" were rare: After more than 1,000 different "administrations" of cannabis over the course of the study, people reported feeling "high" on only three occasions.

Ware said it's possible their plasma levels of THC didn't reach the levels found with recreational smokers. "Street users have access to cannabis that can be anywhere from 15 to 20 per cent THC."