Smoking marijuana associated with higher stroke risk in young adults

Marijuana, the most widely used illicit drug, may double stroke risk in young adults, according to research presented at the American Stroke Association's International Stroke Conference 2013.

In a New Zealand study, and (TIA) patients were 2.3 times more likely to have cannabis, also known as marijuana, detected in urine tests as other age and sex matched patients, researchers said.

"This is the first case-controlled study to show a possible link to the increased risk of stroke from cannabis," said P. Alan Barber, Ph.D., M.D., study lead investigator and professor of clinical neurology at the University of Auckland in New Zealand. "Cannabis has been thought by the public to be a relatively safe, although illegal substance. This study shows this might not be the case; it may lead to stroke."

The study included 160 ischemic stroke/TIA patients 18-55 years old who had urine screens upon admission to the hospital. Among the patients, 150 had ischemic stroke and 10 had TIAs. Sixteen percent of patients had positive drug screens, mostly male who also smoked tobacco.

ASA expert perspective from spokesperson and AHA past president, Ralph Sacco:

Only 8.1 percent of controls tested positive for cannabis in urine samples. Researchers found no differences in age, stroke mechanism or most vascular risk factors between marijuana users and non-users.

In previous case reports, ischemic stroke and TIAs developed hours after cannabis use, Barber said. "These patients usually had no other apart from tobacco, alcohol and other drug usage."

It's challenging to perform prospective studies involving such as cannabis because "questioning stroke and control patients about cannabis use is likely to obtain unreliable responses," Barber said.

In the study, the regional ethics committee allowed researchers to use from other hospitalized patients. But researchers knew only the age, sex and ethnicity for matching due to a lack of consent.

The study provides the strongest evidence to date of an association between cannabis and stroke, Barber said. But the association is confounded because all but one of the stroke patients who were cannabis users also used tobacco regularly.

"We believe it is the cannabis and not tobacco," said Barber, who hopes to conduct another study to determine whether there's an association between cannabis and stroke independent of tobacco use. "This may prove difficult given the risks of bias and ethical strictures of studying the use of an illegal substance," he said. "However, the high prevalence of cannabis use in this cohort of younger stroke patients makes this research imperative."

Physicians should test young people who come in with stroke for cannabis use, Barber said.

"People need to think twice about using ," because it can affect brain development and result in emphysema, heart attack and now , he said.

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Citation: Smoking marijuana associated with higher stroke risk in young adults (2013, February 6) retrieved 18 October 2019 from
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Feb 06, 2013
This is 100% conjecture which is, frankly, disrespectful to your reader base. Tobacco smoking and alcohol consumption are solidly connected by accepted science with a significantly increased risk of stroke... but we think it's the marijuana... I'll believe it when I hear about or see a single person getting a heart attack or stroke from cannabis alone. If your test subjects take other drugs, your data is worthless. They didn't mention the subjects diet or activity level or any other pertinent information about stroke risk factors or possible mechanism. This was a mild assault on my intelligence and the title is misleading. The title is your preferred outcome of a study that you said researchers hoped to conduct in the future. Brown people are made of chocolate pudding, according to research conducted on whales in the southern Pacific... Post an article on that. It's true because I say so. I'm appalled that science with such gaping holes would be spread here of all places on the web.

Feb 06, 2013
But the association is confounded because all but one of the stroke patients who were cannabis users also used tobacco regularly.
"We believe it is the cannabis and not tobacco," said Barber, who hopes to conduct another study to determine whether there's an association...

This article should have been entitled:

Assumptions Based On Belief Systems Associated With Bad Science

Feb 06, 2013
This is not "100% conjecture". It is certainly reasoning under uncertainty, but the data is consistent with the hypothesis that marijuana increases stroke risk. Other explanations also explain the data, but I think the study does a good job justifying more research.

As an aside, almost all empirical scientific research is in some sense "100% Conjecture" since all empirical research involves some uncertainty. Only a few subjects areas have a high enough signal-to-noise ratio for us to be certain, and strictly speaking we still have some probability the data was due to noise.

Feb 07, 2013
"Correlation does not imply causation"
A common logical fallacy which leads to confirmation bias. It is a fundamental flaw in sloppy research and bad science in general. It is already known that nicotine increases stroke risks.

American Stroke Association website:
National Stroke Association website:

But that wasn't even the major flaw here.

Only 16% of the stroke victims tested positive for cannabis, but EVERY SINGLE ONE of them were tobacco smokers. How many of the remaining 84% percent of the non-cannabis stroke victims were tobacco smokers? If anything is relevant here, it is that a large percentage of the stroke victims seemed to have been tobacco smokers! What other common factors did they fail to notice?

Terrible assumptions here! Not scientific thinking at all!

Feb 07, 2013
Clearly the same confirmation bias that leads recreational cannabis users to consider the drug not only harmless but a downright miraculous cure for any ailment, from cancer to the common friggin' cold, can also work in reverse. The day we will all stop and start looking at the effects of recreational drug use objectively - acknowledging that ANY psychoactive chemical compound, regardless of origin, has side effects but could also have potential therapeutic benefits - will probably be too late.

Feb 07, 2013
We are capable of printing organs with "cell ink", why are not able to test what cannabis smoke actually does to human organs?

Feb 07, 2013
Correlation is a necessary condition for causation, so finding it make causation more likely. Statements that use "imply" and "assumption" refer to deduction, which is almost always the inappropriate mode of reasoning for science.

The author of the study is completely transparent about the limitations of the study. And he is CLEARLY pointing out the confounds that the wishful-thinking-pot-smokers on this board point out.

This is how science is done - it is like detective work. Correlation with a confound is like a clue, and it warrants attention and further study. Of course, inferences are quite uncertain when a confound is present: this is the reason more research is warranted.

Not to mention - another clue for causality is temporal proximity. Case studies indicate that strokes happened shortly after marijuana was smoked. Clearly, this supports a causal link b/t pot and stroke, and is much harder to explain away with the confound of tobacco.

Feb 10, 2013
@bizwizSo So if someone has smoked there entire life but just so happens to smoke a joint/ drink a glass of water before a stroke which is most certainly caused by the tobacco... Then that's reason to suspect the joint/glass of water? Talk about not seeing the forest through the trees!

Feb 11, 2013
@Dr Doe -

Your comparison is unfair because it ignores a substantial amount of prior information we have regarding the difference between a joint and a glass of water. Equivalent data on strokes following a joint versus a glass of water ought to be interpreted differently, but not on the basis of the data alone (rather, due to our vast amount of prior information regarding water).

If we assume a small degree of uncertainty regarding the connection between water and stroke, then finding, say, a study of 100 subjects indicates that water increases stroke probability would lead us to increase our belief on the possibility that water causes stroke. But we'd be putting a very small probability on this hypothesis, and it probably can be ignored without much consequence.

Another important note - we could also consider a causal model in which marijuana and tobacco interact (i.e. one moderates the other's causal impact). Certain interactions preclude a clean attribution of cause.

Feb 11, 2013
Also keep in mind that the relationship between two variables in the short-term need not be the same in the long-term. Take exercise and heart-attack. Short-term, exercise increase risk of heart attack. Long-term, it decreases.

If we are seriously considering this hypothesis, then this data obviously does not distinguish between it and the same hypothesis without a long-term benefit.

Our beliefs about these two-hypotheses are determined by our prior information, broadly construed, with a particular emphasis on our causal intuitions about the mechanism behind the pot-stroke relation.

Feb 12, 2013
New Zealand is ranked ninth in the world for cannabis consumption. According to the UN, usage by people 15–45 year-old was 17.9% in 2010. This is slightly more than the stroke victim statistics, and over twice the "baseline" control percentage of 8.1%, which, by the way, was only the percentage of people admitted to the hospital for other reasons, if you read the study. That group was HALF the general population percentage; a flawed number to be comparing.

If only HALF of the general population percentage who use cannabis had to visit the hospital, that would seem to indicate a trend of better health among cannabis users, suggesting a possible health BENEFIT to cannabis, by interpretation of those statistics.

The significant factor that rendered the cannabis users susceptible to strokes appeared to be tobacco, which they ALL used. Only THEN did the percentage of stroke victims match the general population.

Oh, I definitely see the correlation here.

Feb 12, 2013
I am a little unclear about some of the details, but keep in mind that there is a big difference between general health and stroke risk. So the hypothesized "benefit" and "risk" are not mutually exclusive; both could be true.

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