Smoking cannabis from a pipe can significantly reduce chronic pain in patients with damaged nerves, a study suggests.
A small study of 23 people also showed improvements with sleep and anxiety.
Writing in the Canadian Medical Association Journal, the researchers said larger studies using inhaler-type devices for cannabis were needed.
UK experts said the pain relief seen was small but potentially important, and more investigation was warranted.
Around 1 to 2% of people have chronic neuropathic pain – pain due to problems with signalling between nerves – but effective treatments are lacking.
Some patients with this type of chronic pain say smoking cannabis helps with their symptoms.
And researchers have been investigating whether taking cannabinoids – the chemicals within cannabis that effect pain – in pill form could have the same effect.
But the team from McGill University in Montreal said clinical trials on smoked cannabis were lacking.
The study used three different potencies of cannabis – containing 2.5%, 6% and 9.4% of the active ingredient tetrahydrocannabinol – as well as a placebo (dummy version).
Under nurse supervision, participants inhaled a single 25mg dose through a pipe three times a day for five days followed by nine days off, for four cycles.
Those given the highest dose had significantly reduced average pain compared with the placebo as well as less anxiety and depression, and better sleep.
Study leader Dr Mark Ware said: “To our knowledge, this is the first outpatient clinical trial of smoked cannabis ever reported.”
He said larger more long-term studies with higher potencies of cannabis were needed to further test the findings and to better assess safety.
Clinical trials using inhaler-type devices for delivering measured amounts of cannabis should be carried out, he added.
Professor Tony Dickenson, an expert in pain medicine at University College London, said a lot of patients with this type of pain say they benefit from cannabis but there were clearly health issues associated with self-medicating in this way.
He also said the pain relief seen was quite small but could make an important difference to patients who often suffer sleeplessness and depression because of their condition.
It was also worth investigating whether inhaling the drug was a more effective way of getting it into the body than taking it orally, he added.
“It may be important in the future to find patients who respond particularly well because it may be that it is not suitable for some groups, such as older patients,” he said.
“They didn’t get as many patients in the trial as they wanted and it shows that this sort of research is very difficult to do.”
Dr Peter Shortland, a senior lecturer in neuroscience at Barts and The London School of Medicine and Dentistry, said: “Importantly, smoking the drug did not produce the psychoactive effects commonly associated with full strength cannabis.”
He added the trial was “an encouraging step forward” but further large-scale clinical trials were warranted.