Debunking the “cannabis causes high blood pressure” story

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This week saw yet another negative headline in the media about cannabis, and as with almost all the others, it’s complete bullshit. The angle of attack in this instance is blood pressure (hypertension), and the impact cannabis can have on this condition. The original study can be found here.

The Claim

Most articles make the same claim:

“A study has shown that the risk of death from hypertension is higher for cannabis users than for non-users”

The Reality

So what did the paper say, and was the methodology sound? Well, let’s firstly look at how they defined a “cannabis user”:

“We selected participants eligible for mortality follow-up, aged 20 years and above, who answered “yes” or “no” to the question, “Have you ever used marijuana or hashish?” Participants who answered yes were classified marijuana users and those who answered no, as non-marijuana users.”

Oh dear, that’s not exactly a sound classification system. The study is unable to state whether people are heavy users, occasional users, or if they’ve just once had a drag on a joint in high school. This is hugely problematic, and lacks the nuance required to make any solid conclusions, beyond more (better) research being required:

“Recreational marijuana use potentially has cardiovascular adverse effects which needs further investigation.”

That’s not to mention the fact that the study was based on individuals self-reporting their status, which is a famously flawed means of gathering scientific data:

“Specific marijuana use cessation interventions are yet to be documented and self-reported illegal drug use is likely to be under-reported which could have affected the results of our study.”

One study took into account the impact of other factors:

“In multivariate analysis, the associations between marijuana use and systolic blood pressure and triglycerides disappeared, having been mainly confounded by greater alcohol use in marijuana users.”

Another study showed that cannabis can help alleviate high blood pressure:

“We conclude that endocannabinoids tonically suppress cardiac contractility in hypertension and that enhancing the CB1-mediated cardiodepressor and vasodilator effects of endogenous anandamide by blocking its hydrolysis can normalize blood pressure. Targeting the endocannabinoid system offers novel therapeutic strategies in the treatment of hypertension.”

Summary

From the above information we can see that even in the study producing by far the worst conclusion, there are massive flaws in the methodology, meaning that it’s impossible to make claims of causal effect. For instance, it may correlate that people who have high blood pressure are more likely to have tried cannabis to help offset the problem, which would – according to the study – make them appear as somebody whose issues were caused by the plant. It’s the equivalent of assuming that bandages cause cuts, because people with bandages tend to have been physically damaged. It’s presenting a symptom of the issue as if it were the cause.

Moreover, it’s important to remember that scientific consensus comes from a look at ALL studies in the area (meta-analysis), rather than looking at just a single study, so whenever you see outliers such as this, you need to seek out research which either supports or contradicts the postulation, while ensuring that the methodology is sound, and the sample size is large enough. A single study with circa 1200 people is no match for a meta-analysis of all studies ever conducted.

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