When I heard that, it seemed like Canada was the place I should be going to. I packed up my young family and moved to Montreal. What I proposed to McGill was a clinic where we might evaluate the claims of patients about medical marijuana.

So much of what we knew about the drug was anecdotal. Some of it was folkloric. My idea was to listen to the patients’ stories and put them to a clinical evaluation.

When you first moved to Canada in 1999, what was known about medical marijuana?

We certainly knew that cannabinoids were analgesic in animal models. There were case reports floating around of people with multiple sclerosis who’d been helped.

In California, people with H.I.V. were using it for appetite stimulation, nausea and pain. Cancer patients sometimes used it to curb nausea from chemotherapy.

Since then, there have been at least 15 good-quality trials around the world. Cannabinoids are reported to help with H.I.V.-associated neuropathy, traumatic neuropathy, multiple sclerosis, pain from diabetes. There have also been a few small studies on fibromyalgia and PTSD.

When you talk about translational medicine, a drug usually moves from “bench to clinic.” But cannabis has had this unique trajectory: The patients were using it on their own, and then you had these papers, often based on a few case studies. And sometimes, you had later trials which led to drugs — like with H.I.V. patients’ using cannabis, which led to Marinol.

Tell us about some of your own research.

One investigation we published in the Canadian Medical Association Journal in 2010 studied 23 patients who used three slightly different levels of cannabis preparations and one placebo for two months. They had one puff three times a day. We found that the 9.4 percent THC level was superior to the placebo in terms of its effect on pain.