Does Cannabis Increase The Risk Of Stroke?

One recent meta-analysis study appears to have found a link between cannabis and stroke.  And even go so far as to claim that causation is likely. But there is less here than meets the eye, once again.  To quote the abstract of the study:

Meta-analysis demonstrated significant associations of cannabis (OR = 1.37, 95% confidence interval (95% CI) = 1.14–1.65), cocaine (OR = 1.96; 95% CI = 1.27–3.01), and amphetamines (OR = 2.22, 95% CI = 1.40–3.53) with increased stroke risk, while no significant association was observed for opioids. Findings for cannabis showed some heterogeneity and small-study effects. MR analyses revealed that cannabis use disorder was associated with any stroke (OR = 1.11 [1.01–1.51]) and large artery stroke (OR = 1.35, 95% CI = 1.01–1.80), and cocaine dependence was associated with cardioembolic stroke (OR = 1.08, 95% CI = 1.02–1.14) and intracerebral hemorrhage (OR = 1.38, 95% CI = 1.15–1.65). Genetically predicted substance use disorder overall was associated with any stroke (OR = 1.33, 95% CI = 1.02–1.72) and intracerebral hemorrhage (OR = 7.79, 95% CI = 3.46–17.54). Problematic and dependent alcohol use was linked to large artery and cardioembolic stroke, whereas nicotine dependence showed no significant associations.

While this is pretty good confirmation of one of the many notorious risks of using/abusing cocaine and amphetamines, and to a lesser extent problematic and dependent alcohol use, for cannabis the results were actually pretty weak by good epidemiological standards in terms of the odds ratios (which were even weaker still for younger people).  Remember, a "meta-analysis" is simply a study that looks at many other studies and pools them all together, with all of the pitfalls that entails, and there was a lot of "heterogeneity" among studies to put it mildly, including several studies with null (i.e. statistically insignificant) or even "wrong" signed results mixed in.  And the reddest flag of all?  The null result for nicotine (i.e. tobacco) dependence flies in the face of what has long been known about the very real and dose-dependent health risks of nicotine, or at least smoking tobacco (a two-, three-, four-, or even six-fold increase in stroke risk depending on how much one smokes).  Even vaping nicotine may not be so benign after all in terms of stroke either.  That's like that old study that found that pork consumption was significantly associated with cirrhosis of the liver, but that alcohol was no longer significant once pork was adjusted for.

In other words, selection bias, reporting bias, residual or unmeasured confounding, "thumb-on-scale syndrome", and other statistical quirks could very easily be the reasons for these findings.  To say nothing of the pitfalls of both meta-analyses and Mendelian randomization as well.  The ever-insightful John Ioannidis would likely have a field day with this one!

So what should be the takeaways from this study? If you don't wish to have a stroke anytime soon:

• Don't use cocaine or amphetamines.  Or if you do, the less you use, the better.
• If you choose to drink, do so in moderation.
• Regardless of what one particular study says, the weight of the evidence says that if you smoke tobacco or otherwise use nicotine, you should quit ASAP.  Better yet, don't start!
• If you can't quit nicotine entirely just yet, at the very least quit smoking. "The more you smoke, the more you stroke".
• Opioids may not increase the risk of stroke, but they can kill you in other ways, so beware.
• If you are at high risk for stroke yourself, be extra careful about what substances you use, and how much and how often.  Seriously!
• When it comes to cannabis, less is more.  There is not a chronic, heavy user alive that would not benefit from at least cutting back a bit.  Think of it as a spice, not a food.
• You don't need to leave the solar system every time you get high either, by the way.  Try a more "down-to-earth" approach next time. 
• Remember that there are a number of other ways to use cannabis that don't involve smoking.
• Above all, don't panic!  You really need the extra stress like a hole in the head!  Which can take on a rather literal significance here.

And there you have it.  Stay healthy, folks!