Puffing cannabis from a pipe can significantly reduce chronic pain in patients with damaged nerves, a study has shown.
Smoking the drug also lessened anxiety and depression, said researchers.
Pill preparations of cannabis extract have previously been successful in treating certain types of pain.
But researchers avoided studying the effects of smoking cannabis, as if taking the drug to get high.
A team of Canadian scientists conducted a trial to test inhaled cannabis in 21 patients with chronic neuropathic pain caused by traumatic injury or surgery.
The researchers used herbal cannabis, or "grass", at four potency levels. Patients aged 25 to 77 were asked to smoke 25 milligrams of the drug from a pipe three times a day.
The highest-potency cannabis, containing 9.4% of the active ingredient THC, reduced pain, decreased anxiety and depression, and aided sleep.
Lead researcher Dr Mark Ware, director of clinical research at the Alan Edwards Pain Management Unit, McGill University Health Centre, said: "We found that 25mg herbal cannabis with 9.4% THC, administered as a single smoked inhalation three times daily for five days, significantly reduces average pain intensity compared with a 0% THC cannabis placebo in adult subjects with chronic post-traumatic/post-surgical neuropathic pain."
No serious adverse effects occurred during the trial. All the patients taking active cannabis reported one occasion of feeling "high" and euphoric, and some complained of headaches, dry eyes, a burning sensation, dizziness, numbness and coughs. No significant changes in vital signs, heart rate or kidney function were recorded.
The findings, reported in the Canadian Medical Association Journal, CMAJ, are believed to be the first from a clinical trial of smoked cannabis. The authors recommended more studies with higher potencies of THC, longer follow-up periods, and flexible dosing.