A recent meta-analysis published in the journal Heart made headlines across the world with claims that cannabis use more than doubles the risk of dying from cardiovascular disease. But before we panic about our evening joint or gummy, it’s worth taking a closer look at what this research and other studies actually tell us, and more importantly, what they don’t.
The research, which analysed 24 studies from 2016 to 2023, concluded that cannabis users face a 29% increased risk of heart attack and a 20% increased risk of stroke. These are certainly attention-grabbing numbers, but the devil, as they say, is in the details, and the study has a number of flaws.
The problem with proving causation
Here’s the first red flag: 19 of the 24 studies included were cross-sectional studies. Think of these as snapshots in time. They can show us associations between cannabis use and heart problems, but they can’t prove that cannabis actually caused those problems. It’s a bit like noticing that people who carry umbrellas tend to get wet and concluding that umbrellas cause rain.
Cross-sectional studies are notoriously weak when it comes to establishing causality, which is exactly what this meta-analysis is trying to do. To really understand whether cannabis causes heart problems, we need longitudinal studies that follow people over time, watching their health outcomes develop. The fact that most of this research relies on these weaker study designs is a significant limitation.
The data quality dilemma
Perhaps even more concerning is how cannabis use was actually measured in these studies. The meta-analysis authors themselves acknowledged that “cannabis exposure was poorly reported in included studies, preventing meta-analysis assessment.” In plain English: the researchers couldn’t properly combine the data because different studies measured cannabis use in completely different ways.
Some studies looked at any lifetime use, others focused on recent use, and many failed to distinguish between medical and recreational consumption. Most crucially, the research doesn’t separate different consumption methods – smoking, vaping, edibles, or oils – despite these having vastly different effects on the cardiovascular system.
The bias problem
The research quality issues don’t stop there. Many of the included studies had what researchers call “moderate to high risk of bias due to missing data information.” This means crucial information about participants was often incomplete or missing entirely.
There’s also the issue of misclassification, particularly in studies that relied on medical databases to identify cannabis users. These databases are notoriously bad at capturing non-medical drug use, since people often lie or misreport substance use, especially in countries where they are breaking the law. This creates a systematic bias where only certain types of cannabis users are identified and studied.
Recent analysis in the Harm Reduction Journal highlighted how this under-reporting due to legal concerns and stigma significantly skews research results, creating selection bias toward sicker populations in medical databases.
Double-counting concerns
Several studies in the meta-analysis used overlapping data sources and time periods. This raises the possibility that some patients were counted multiple times across different studies, artificially inflating the apparent strength of any association.
It’s a bit like surveying the same group of people multiple times and then claiming you’ve gathered evidence from a much larger population than you actually have.
The confounding factor elephant in the room
Perhaps the most glaring issue with current cannabis cardiovascular research is the failure to adequately control for tobacco use. Cannabis users are significantly more likely to smoke cigarettes or mix tobacco into a joint, yet many studies fail to separate the risk factors from this additional substance.
This “lifestyle clustering” means cannabis users often have multiple cardiovascular risk factors that studies struggle to untangle. Without proper controls for tobacco use, diet, exercise habits, and healthcare utilisation patterns, it’s nearly impossible to determine what role cannabis itself plays in any observed health outcomes.
When the evidence points the other way
Interestingly, not all recent research supports the doom-and-gloom narrative. A 2008 study found that low-dose THC may actually have anti-inflammatory effects that could protect against atherosclerosis, the buildup of fatty deposits in arteries that leads to heart attacks and strokes.
In a 2017 study, researchers aimed to measure the effect cannabidiol (CBD) had on blood pressure (hypertension) in healthy volunteers. Nine male participants were given a single 600 mg dose of CBD or placebo in a randomised, placebo-controlled, double-blind, crossover study design. The researchers concluded that “resting blood pressure was lower after subjects had taken CBD and that CBD blunted the blood pressure response to stress”. Meanwhile, a 2023 cohort study of 91,161 volunteers in the UK found that both male and female cannabis users experienced lower overall blood pressure than non-users.
These findings directly contradict claims that all cannabis use increases cardiovascular risk.
The reverse causation problem
There’s another methodological issue that researchers call “reverse causation”, the possibility that people with existing health problems are more likely to use cannabis, not that cannabis causes health problems.
Medical cannabis patients, by definition, already have health conditions that may put them at higher cardiovascular risk. When studies find associations between cannabis use and heart problems, it might simply be because people with health issues are more likely to be using cannabis for symptom relief, not because cannabis caused their problems in the first place.
What this means for cannabis users
None of this is to say that cannabis has zero effects on cardiovascular health; like any psychoactive substance, it can have physiological impacts. But the current evidence simply isn’t strong enough to support the dramatic claims making headlines.
The fundamental problem is that most research in this area suffers from the same limitations: poor study design, inconsistent measurements, and an inability to account for confounding factors like tobacco use, diet, exercise, and other lifestyle choices that significantly impact heart health.
Leading cardiovascular researchers have increasingly called for randomised controlled trials with standardised cannabis preparations, better separation of consumption methods in analyses, and longitudinal studies that adequately control for tobacco use and other confounding factors.
The bigger picture
For medical cannabis patients, this research adds little to our understanding of actual risks. The studies don’t distinguish between different consumption methods (smoking vs vaping vs edibles), dosages, or cannabinoid profiles, all factors that could significantly impact any potential cardiovascular effects.
The complexity of cannabis as a substance, with multiple active compounds, various consumption methods, and diverse user populations, makes simple risk assessments problematic without more rigorous research designs.
What we need is better research: well-designed longitudinal studies that follow people over time, with consistent measurements of cannabis use and proper controls for other risk factors. Until we have that, claims about cannabis doubling death risks from heart disease remain scientifically premature.
The truth about cannabis and heart health isn’t found in sensationalist headlines, but in the methodological details that often get overlooked. And those details suggest we’re still a long way from having definitive answers.
If you are a medical cannabis patient concerned about cardiovascular risks, the best approach remains to consult with your clinic. They can help assess individual risk factors and make informed recommendations based on your specific circumstances.
