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Medical Weed Is Safe, Longest Ever Study on Managing Pain with Pot Finds

“It should certainly offer some reassurance for medical legalization.”
Image: Flickr/Brett Levin

The results from the first and longest study ever conducted on medicinal cannabis use for chronic pain management are finally in: medical pot is safe to use. Nobody's brain turned into a fried egg.

The study was conducted by researchers at McGill University in Montreal and followed 215 experienced smokers as they toked up 2.5 grams a day, as well as 216 non-smokers, for four years—between 2004 and 2008, with a follow up after one year. The participants, who all suffered from a wide range of noncancer chronic pains, were tested for lung function, underwent blood tests, and had cognitive abilities like memory examined. All adverse effects, from mild headaches to serious disorders, were reported by subjects.

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In a paper describing the results of the study, published Tuesday in The Journal of Pain, the researchers write that the cannabis users reported less pain, better overall moods, and had no increased risk of serious adverse effects compared to the control group. Cognitive functions like memory, the researchers wrote, "improved in both groups."

"This is a useful reality check"

"The results suggest that cannabis at average doses of 2.5g/d in current cannabis users may be safe as part of carefully monitored pain management program when conventional treatments have been considered medically inappropriate or inadequate," the authors wrote.

That's not to say frequent smoking didn't have any drawbacks at all. Compared to the control group the experienced smokers reported more non-serious adverse effects, such as dizziness and mild respiratory issues associated with smoking.

"Respiratory illnesses and other minor side effects are real risks, but they're much more modest (in my opinion), than some of the risks that people believe exist," Dr. David Casarett, a physician at the University of Pennsylvania and author of the measured Stoned: A Doctor's Case for Medical Marijuana, wrote me in an email. "So this is a useful reality check."

Despite all the controversy surrounding the legalization of medical marijuana in the US—it's already legal in Canada—most research has focused on recreational marijuana use, Casarett said. Medical marijuana remains understudied. Some of these studies about recreational use focus on the supposed harms, like a recent paper that concluded casual cannabis use can cause brain abnormalities. It was later found to have serious methodological shortcomings.

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"This study was of people using medically, for one very specific indication (pain)," Casarett continued. "It should certainly offer some reassurance for medical legalization."

The McGill study has several limitations, the authors note, that should be considered by anyone who might be thinking that it covers all pot use. One year may not be enough time to catch all serious adverse effects, for example. Most of the subjects were also experienced users, and the pot they smoked was 12.5 percent tetrahydrocannabinol (THC, marijuana's main psychoactive property)—no more, no less. Just try and ask your dorm room dealer what the exact THC content of the skunk you just picked up is and see what they say. Probably, they won't know, and THC levels in weed can range from around 10 percent to nearly 30, if grown by the most meticulous producers.

The study's results are thus only applicable to experienced cannabis smokers managing chronic pain. Regardless, marijuana is a chronically under-researched drug—we don't even really know what causes one of pot's most well-known side effects, the munchies. A study of this size is an important contribution to our current body of knowledge about how safe (or unsafe) it really is to get high.