Cannabis and Cardiac Risk

Two preliminary studies looking at the effects of cannabis on cardiovascular events are being presented at the American Heart Association’s Scientific Sessions 2023, Nov 11-13 – let’s take a look at what these studies found and what we should take away.

The first study is “Daily marijuana use is associated with incident heart failure: ‘All of Us’ Research Program” (Poster Abstract MDP250). More than 150,000 adults completed a survey that included questions about cannabis use and then they were followed for up to 4 years. Approximately 2% (2,958) of those people developed new heart failure during those four years, and people who noted that they used cannabis daily had a 34% increased chance of developing heart failure  compared to people who didn’t use it at all. Of note – “cannabis use” was defined here as non-medical use, or use that exceeded the dose needed for medical purposes.

The other study is “Increased risk of major adverse cardiac and cerebrovascular events in elderly non-smokers who use cannabis” (Poster Abstract MDP249). This study drew from a large database of information on people who had been hospitalized. It looked at whether people who were documented to have cannabis use disorder and risk factors (such as high blood pressure, high cholesterol, and diabetes) had an increased risk of having a heart attack, stroke, or other bad cardiac event while they were admitted. The results showed that folks with cannabis use disorder did have a higher chance of having a cardiac event while hospitalized – for example the rate of having a heart attack during the hospital stay was 6% vs 7.6%.

There are certainly good reasons to want to know more about the potential adverse cardiovascular effects of cannabis use. Under “cannabis” there are a variety of cannabinoids that will interact with many different types of receptors found in the cardiovascular system, and which lead to complex and sometimes opposite effects.

THC in particular does interact with CB1 receptors within the heart and blood vessels and seems to lead to elevated heart rate, elevated blood pressure, and perhaps reduced contractility of the heart muscle. In people with known heart disease or risk factors this definitely could lead to an increased chance of bad outcomes like heart attack, stroke, abnormal heart rhythms, and heart failure. There are also reports of cannabis possibly leading to coronary vasospasm and more research needs to be done on cannabis’ effects on things like platelet clotting and atherosclerosis.

So what should we think about these studies? Well, they are observational studies – so the investigators are not able to keep control over all the factors that they would in something like a randomized control trial. However, studies like this let you get a much larger sample size – it would  be extremely (and likely prohibitively) expensive to try and have a randomized control trial of 150,000 patients looking at cannabis. A study design like this can sometimes mean that unknown confounders can come into play, where the outcome is not related to cannabis use itself, but to another hidden factor that just happens to correlate with cannabis use. In the heart failure study though, it sounds like they did a very thorough job of doing a mathematical adjustment to account for the most obvious confounders like socioeconomic status, alcohol and tobacco use, and cardiovascular risk factors.

I think the biggest thing to consider about how to apply these studies is how they define cannabis use. In the heart failure study it was daily non-medical cannabis use. In the hospitalization study it was people who were diagnosed with cannabis use disorder (which is somewhat problematic as there is not a set definition and different healthcare systems will use different criteria to determine it) – and overall it also implies likely non-medicinal use. This is completely an educated guess on my part, but most of the patients included in these studies are likely using high dose THC recreationally. We don’t have any information on how many milligrams of THC they are using or where they’re getting it. Are they buying it from licensed and reputable dispensaries or are they getting it from the black and gray market – where there’s a higher chance of contamination with other drugs, heavy metals, and fungal or bacterial contamination? Do they think that they are using delta-9 THC (the molecule that is found naturally in the plant) but are actually consuming a synthetic derivative such as Delta-8 or THC-O (which are molecules that we know much less about in terms of their effects on health)? Nor do we have any information on how they are consuming it – are they smoking it, vaping it, or are they consuming it via edibles? Again this is an educated guess, but I would imagine that most of these study participants are likely inhaling it via smoking – and consuming it that way could increase the risk of some of these cardiovascular outcomes separate from the effects of the cannabis itself.

When medical patients use cannabis they are using it very differently. Significantly more medical patients choose edible preparations as the method of ingestion. Also the amount of THC used for many medical patients is much less and usually is done in conjunction with other cannabinoids such as CBD, CBG, CBDa and others. In addition to just using lower doses of THC, there is also good evidence showing that CBD for example modulates the effects of THC on the cardiovascular system (in one study using CBD significantly reduced the amount that the heart rate would elevate when the subject was then exposed to THC). (https://doi.org/10.1007/s00406-019-00978-2)

Overall these studies are providing information that is really good to know, and I’m glad that we have them. But we are also in need of bigger and more robust studies, especially prospective studies to look at cannabis use under medical guidance to see if there are still these same risks. You could imagine the difference between somebody using one aspirin a day as prescribed in order to reduce the risk of having another heart attack versus somebody who is using 10 or more – they have gone beyond the therapeutic dosage and are opening themselves up to other dangerous side effects like severe bleeding. Similarly there’s a big difference in the risk between someone drinking a half glass of wine a night and somebody drinking one to two bottles.

And as a counterpoint to these studies there was a systematic review published in 2023 (https://doi.org/10.1016/j.toxrep.2023.04.011) that combined the results of 20 published observational studies, which included between them more than 180 million patients. And the analysis of these combined studies did not show a significant increase in the risks for adverse cardiac outcomes in people who use cannabis. (The caveat to a study like this is that it’s combining a bunch of different studies who have different methodologies.) But it highlights that we need more research to know more precisely how cannabis affects the risk of cardiac events.

So in terms of big picture takeaways: everyone should talk to their health care providers about their cannabis use (assuming you feel safe to do so), especially if they have known cardiovascular disease or risk factors for it such as hypertension, high cholesterol, or diabetes. And if you have these risks and use or are interested in using medical cannabis, please seek out a trained and experienced cannabis clinician that can help educate and guide you to use cannabis in a way that is both safe and effective.

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