The Deadly Health Risk Most Pot Smokers Don’t Know About

New research shows that marijuana doubles the risk of stroke—and not in older people.
February 15, 2013

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Shari Roan is an award-winning health writer based in Southern California.

Though still illicit, marijuana has always retained a hazy reputation as being fairly "safe." But that idea has taken a hit (so to speak), with new research showing pot can raise your risk of a stroke.

The study, out of New Zealand, found that patients who had either an ischemic stroke or transient ischemic attack (TIA; also known as a "mini-stroke") were over twice as likely to have cannabis in their urine compared to people who did not have a stroke. A stroke happens when blood flow is impaired to part of the brain.

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The study challenges the assumption that there are few serious risks to using marijuana, says the lead author of the paper, Alan Barber, a professor of clinical neurology at the University of Auckland. "Marijuana is associated with lung disease, heart attacks, and atrial fibrillation," in which the speed or rhythm of the heartbeat is impaired, Barber told TakePart. "[Marijuana] is also known to constrict down the arteries in the brain. Our study suggests that a lifestyle that includes marijuana is associated with an increased risk for stroke as well."

The research was presented recently at the American Stroke Association's International Stroke Conference 2013. The study included 160 people who'd had an ischemic stroke or TIA. The patients were comparatively young—ages 18 to 55—who'd had their urine screened when they were admitted to the hospital. Sixteen percent of the stroke patients tested positive for pot. In comparison, about eight percent of those in a control group (meaning people who didn't have a stroke/TIA) tested positive for cannabis in urine samples.

Most of the stroke patients who had positive marijuana urine tests also smoked cigarettes; they had no other risk factors for stroke, other than tobacco, alcohol, or drug use. Barber said that because tobacco exposure also increases the risk of stroke, it can be difficult to separate out the effects from marijuana smoke. However, he think marijuana smoke confers an additional risk because of case reports that have linked marijuana use to stroke.

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The link between marijuana and stroke had not been made before because doctors probably didn't think to ask about drug use in stroke patients, he says. "If you don't look for something you won't find it," Barber says. "Also, the absolute risk of stroke for someone in their 20s or 30s is very low. A doubling of this risk still results in a low overall risk, so it's easy to miss. However, the average age of our patients was 45 years. The risk of stroke in this age group is moderate. A doubling of a moderate risk is clinically significant."

It's not clear why smoking pot increases the risk of stroke, Barber says. But there are lots of plausible theories: For starters, cannabis affects the heart and blood vessels in a number of ways. Most strokes are caused by a clot that forms in the heart or one that breaks off from the wall of an artery and then moves with the blood to the brain. Marijuana also increases heart rate and blood pressure and adds to the workload of the heart as it pumps blood to the body.

Previous research has linked marijuana use to heart attack risk—a fivefold increase of heart attack in the hour after using marijuana. Smoking pot can also cause heart palpitations and heart rhythm disturbances such as atrial fibrillation (an irregular, rapid heart rhythm). Both heart attack and atrial fibrillation increase the risk of stroke.

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Marijuana has also been shown to increase the risk of a spasm of the arteries supplying blood to the brain. Barber advises people who have had a stroke or who are at high risk for stroke to avoid lighting up. And doctors should test younger stroke patients for marijuana use, he says. "People need to think twice about using cannabis," because it can affect brain development and result in emphysema, heart attack, and now stroke, he says.

Do you think marijuana is bad or good for health? What’s your opinion on the use of medicinal marijuana for some ailments?