A Kingston cardiologist and former president of the Canadian Medical Association will be sounding the alarm on Friday at a major meeting of stroke experts over the link between marijuana and heart attacks and stroke.

“I’m not here to demonize cannabis. We don’t need to go back to reefer madness. But we can’t say it does not harm,” said Dr. Chris Simpson, who was president of the CMA in 2014 and 2015 and made the public health implications and possible medicinal uses of marijuana one of the focal points of his term.

“If we have several million people smoking marijuana every day, we want to know what the effects are,” said Simpson, who will be addressing researchers, health professionals and policy-makers at the Canadian Stroke Congress.

There is a widespread belief that marijuana is actually healthy, yet someone who has smoked a joint is about as likely to have heart attack within an hour as someone who has shovelled snow, said Simpson.

The higher the THC — the psychoactive compound — in marijuana, the higher the risk, he said. THC activates certain receptors in blood vessels. When THC binds to these receptors, it leads to the buildup of plaque and makes blood more “sticky,” which leads to the creation of blood clots. “In the case of a stroke, a blood clot lets go and travels to the brain.”

Each marijuana joint has 70 cannabinoids of unknown dose. The way that these chemicals interact with the endocannabinoid system is unpredictable, said Simpson.

That’s not to say there isn’t potential uses for some of these chemicals. But there’s a lot that’s still unknown, he said. And it’s hard to tease out marijuana’s effects on health because people who smoke marijuana often also smoke cigarettes.

“When we study a new drug, like a statin, for example, it is a single drug — a single molecule. We know what receptors it works on, why it works, how it works, and have tested it extensively under rigorous conditions to ensure it is both safe and effective. To expect 70 chemicals of unknown dose to exert a specific effect, or even a net positive effect, is magical thinking,” he said.

“From a public health perspective, the balance of evidence so far is that using marijuana is not good for your heart, blood vessels and brain.”