News release: ‘‘Cannabis may be legal, but we need to address the public health impact’’

It’s not the harmless, natural product it’s been marketed as, says Dr. Simpson of Queen’s University

(Ottawa) — Although most Canadians support the legalization of cannabis, not nearly enough attention has been devoted to its harmful health effects, says cardiologist Dr. Chris Simpson, Vice-Dean (Clinical) in the Faculty of Health Sciences at Queen’s University and professor of medicine.

Dr. Simpson told the Canadian Stroke Congress that every marijuana cigarette has a mixture of 70 chemicals -- all of which may act on the heart and blood vessels in unpredictable ways. 

For stroke clinicians, there is particular concern because the psychoactive chemical THC attaches to receptors in the brain and blood vessel walls that can promote plaque growth in the arteries. In addition, it can make the blood sticky, causing platelets to stick together and create clots. Atherosclerosis (clogged arteries) is linked to vascular diseases like stroke, heart disease, and heart failure.

“The degree of knowledge we have about the potential benefits and harm of cannabis is really in its infancy,” Dr. Simpson says. “But the harm seems to greatly exceed any potential benefits.” He says there needs to be more thorough study of the impact of cannabis on vascular health. 

“There is this huge popular misconception about cannabis, driven by social media and the cannabis industry, that it is a medicine or it is harmless,” Dr. Simpson says. “I worry when people say that cannabis is a natural product that has been used for a thousand years and so it must be safe. Countering that belief is tough. There are huge gaps between what people believe and what is reality.”

Cyclic vomiting syndrome

Among concerns, doctors are seeing “a really mysterious condition in habitual users” called Cyclic Vomiting Syndrome that leads to incessant vomiting and dehydration, particularly in young adults. “One of the very peculiar features of diagnosis is that the only thing that relieves the vomiting is a hot shower,” Dr. Simpson says. Treatment involves anti-nausea drugs and waiting for the cannabis to clear their system. “It can be very dangerous because it messes up electrolytes, may cause potassium depletion, and lots of harm can come from dehydration.”


A former president of the Canadian Medical Association, Dr. Simpson also expressed concerns about edibles, which will be legalized later this month. The absorption rate of THC is much slower with edibles so people may consume too much before feeling the high, resulting in ingesting way more than they should.

“It hits you like a ton of bricks at the one-hour mark or so and you can overdose very quickly because of the delayed reaction,” he says. Dr. Simpson is particularly worried about accidental ingestion of edible products by children.

He also highlighted evidence that cannabis affects cognitive development up to age 25, and its use can trigger psychoses and aggravate anxiety and depression. He questioned legalization at age 19, saying policy-makers “haven’t fully considered all the negative consequences and risks posed to our young people in particular.”

While it is best to avoid cannabis use, for those who do he recommends very moderate use, not smoking or not inhaling deeply, caution with edibles, starting use of the product as late in life as possible, and staying away from synthetic cannabis which is potent and very dangerous.

Other hot topics today: 

Future shock: Stroke experts probe ways to prevent stroke, promote recovery as health system braces for flood of boomers. Two UK stroke experts – one focused on prevention and the other on recovery – warned Canadian stroke clinicians that something’s got to change to meet the growing need for stroke services. With increasing numbers of Boomers experiencing stroke and more than 80 per cent of people surviving, stroke has become a disease of chronic disability. “Ninety per cent of strokes could be prevented with better monitoring of blood pressure, better detection of atrial fibrillation and better followup after TIAs (mini-strokes),” says stroke neurologist Dr. Peter Rothwell of the University of Oxford. “We need to be more efficient and effective in managing the risk factors that we already know about and more effective in using treatments we already have.” At the same time, more investment is needed in stroke recovery because survival rates are increasing and more people are living with long-term disability. (Canada Hall 1, 3:30 to 4:30 pm)

Caring for carers: Family caregivers provide essential support to stroke survivors but many experience stress that can compromise their health. Interventions to decrease caregiver stress have demonstrated limited benefits. A new study from the University of Toronto, led by Dr. Jill Cameron, looked at data collected on more than 700 caregivers at 1, 3, 6 and 12 months post-stroke. The study found that ability to cope with caregiving challenges is directly related to the mental and physical health of the caregiver and, as such, more effort is needed to provide support. (Room 202, 10:30 am -12:00 pm)

Stroke and women: Heart & Stroke staff reviewed more than 1,000 online community discussion posts and toured the country to talk to 200 women who experienced stroke to develop a plan to address systemic disparities in stroke care. At a special session on Saturday morning (Oct. 5), participants will hear the perspectives of a woman who experienced a stroke, hear about unique considerations in rehabilitation for women, and learn about new approaches to address gender issues. (Saturday, Canada Hall 1, 8:35-10:00 am)

The Congress is being held in Ottawa, ON from Oct. 3-5, 2019. Follow us on Twitter @strokecongress, #StrokeCongress.

Statements and conclusions of study authors are solely those of the study authors and do not necessarily reflect H&S or CSC policy or position. Heart & Stroke and the Canadian Stroke Consortium make no representation or warranty as to their accuracy or reliability.

Canadian Stroke Congress

Co-hosted by Heart & Stroke and the Canadian Stroke Consortium, the Canadian Stroke Congress is a uniquely Canadian forum for experts to share the latest research findings, exchange ideas, and make the connections which will change the future of stroke. It brings together researchers, neurologists, nurses, rehabilitation specialists, policy makers, health system decision makers – and many others – in an unprecedented opportunity to improve the brain health of Canadians. (

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Canadian Stroke Consortium

The Canadian Stroke Consortium is the professional organization for stroke neurologists, leading continuing education, advocacy and research for healthcare professionals. (

For story ideas, the embargoed Congress media releases and media interviews:
Cathy Campbell
Heart & Stroke