TORONTO -- Medical cannabis appears to be reasonably safe as well as effective in helping to relieve chronic pain -- at least in patients who have previously used the drug, a study has found.
The benchmark study, conducted at seven pain-management clinics across Canada, found that patients who used medical marijuana to treat chronic pain had a similar level of serious side-effects as those not using the herb.
Principal investigator Dr. Mark Ware, a pain specialist at the McGill University Health Centre, said researchers found no difference in the rates of serious pot-related adverse events -- including the need for hospitalization or death -- between the two groups during a year of monitoring patients.
The study did find that patients in the cannabis group had an increased rate of mild to moderate side-effects compared to those in the non-using control group, such as headache, nausea, drowsiness and dizziness, but those were expected, Ware said Tuesday from Montreal.
Researchers also measured changes in pain, mood and quality of life among the patients, whose chronic pain arises from such conditions as arthritis, fibromyalgia, multiple sclerosis and spinal cord injury.
"We found that both groups improved over time," he said. "But the cannabis users actually improved significantly more than the control group."
The 2004-2008 study, published online in The Journal of Pain, followed 215 adults with chronic non-cancer pain who used medical cannabis and compared them to a control group of 216 chronic pain sufferers who were not marijuana users. In both groups, conventional pain therapies had not been effective.
The cannabis group was given access to dried marijuana from a licensed producer, which contained 12.5 per cent of the active ingredient tetrahydrocannabinol, or THC.
On average, patients used about 2.5 grams of the medical-grade pot per day, in line with recommendations under Health Canada's Marihuana for Medical Purposes Regulations. The drug was smoked, vaporized or ingested as an edible.
Ware said neurological assessments showed cannabis users had no adverse cognitive effects, and blood tests on a subset of the patients showed no ill-effects on their kidney, liver or hormonal function.
However, pulmonary tests showed those in the pot group had some restriction in lung function after one year -- a change that could be compounded over time and lead to more significant impairment, Ware speculated.
"What we couldn't do was separate out the effects of tobacco," he said, noting that many of the marijuana users also smoked tobacco or mixed it with the medicinal weed.
"We couldn't tease out what was just a pure cannabis smoking effect and what was tobacco. So we recommend caution in terms of smoking."
While the findings can't be directly extrapolated to first-time marijuana users, Ware said they can help doctors decide whether the drug might work on a particular patient's chronic pain.
"What this now does is it gives the clinician an opportunity to review with the patient, based on real evidence, what are the possible side-effects they may have if you use this therapy and what are some of the possible benefits," he said.
With eligible patients now able to purchase medical cannabis from 25 licensed producers with a doctor's authorization, such evidence-based guidance is "desperately" needed, stressed Ware.
"That's the big stumbling block that every clinician has: 'I just don't know what to tell my patients.' Well, now you have a detailed listing ... and (doctors) can read through every possible side-effect."
The study, along with supplementary material, will be published in the print version of the journal in mid-October.
