Habitual cannabis users had about 9% fewer extra heartbeats on days they were randomly assigned to inhale cannabis than on days they were assigned to abstain, according to a trial of 108 adults published Aug. 12 in the Journal of the American College of Cardiology. The UC San Francisco team that ran the study had predicted the opposite, and its senior author says the result should not be read as evidence that cannabis is good for the heart.
Key takeaways
- Cannabis days came with about 9% fewer extra heartbeats, driven by extra beats starting in the heart’s upper chambers.
- Extra beats from the lower chambers did not change, and neither did daily steps, sleep duration, or glucose.
- This measured hours, not years. It says nothing about long-term heart risk from smoking or vaping cannabis.
What the study found
The trial, called MARY-JANE, enrolled 108 adults who smoked or vaped cannabis regularly and had no history of a diagnosed heart rhythm problem, according to the study abstract. Their average age was 32, and 59% were women. For 14 days, a text message told each person whether that day was a cannabis day or an abstinence day. The assignment covered 713 cannabis days and 616 abstinence days across the group.
Everyone wore a patch monitor recording every heartbeat, plus an activity tracker, a sleep monitor, and a continuous glucose monitor. Each person served as their own comparison, which cancels out most differences between people.
Total extra beats, called cardiac ectopy, fell about 9% on cannabis days (rate ratio 0.91). The real effect is very likely somewhere between 2% and 15% fewer (95% CI 0.85 to 0.98), and there is roughly a 1 in 50 chance a difference this size came from luck alone (P = 0.02).
The drop came almost entirely from premature atrial contractions, the extra beats that start in the upper chambers. Those fell about 10% (rate ratio 0.90), very likely between 3% and 18% fewer (95% CI 0.82 to 0.97). Premature ventricular contractions, which start in the lower chambers, showed no significant change.
The researchers also saw no significant differences in daily step counts, sleep duration, or glucose levels.
“We were surprised by the results because they contradicted our original hypothesis,” said Gregory Marcus, MD, MAS, a professor in the UCSF Division of Cardiology, in the university’s news release. He added that “the overall health effects of inhaled cannabis may still be net negative.”
Dr. Kumar’s take
A premature atrial contraction is a marker, not a disease. Its frequency is one of the strongest statistical predictors of who later develops atrial fibrillation, which is why cardiologists count them. Pushing that count down for a few hours in people already used to THC is not the same as lowering anyone’s risk of atrial fibrillation, stroke, or vascular injury. Nobody has shown that moving the marker moves the outcome.
The trial’s own authors list the reasons to hold this loosely: adherence to the daily assignment was only moderate, other things people did on those days could not be controlled, and withdrawal on abstinence days may be doing some of the work. That last one matters. If skipping cannabis makes a regular user restless and irritable, the abstinence day may be the abnormal day, and the comparison drifts from “cannabis lowers ectopy” toward “not using raises it.”
The result worth more attention is sitting in the secondary endpoints. Cannabis moved nothing on sleep duration, step counts, or glucose. That is the opposite of what most regular users would tell you it does for them, especially for sleep. Measured sleep duration matters for real endpoints, including how the body handles glucose, as in this look at weekday sleep duration and estimated glucose.
Also worth saying plainly: this trial studied inhaled cannabis over 14 days, and what it counted was extra heartbeats. Anything else inhaling might be doing to the body was outside what this study measured.
What it means for you
Nothing here is a reason to start using cannabis, and nothing here is a reason to stop on cardiac grounds if you already do. A short-term shift in a rhythm marker is too thin a basis for either decision.
If you feel skipped beats, flutters, or a racing pulse, that deserves an evaluation on its own, not a self-experiment with cannabis. And if you use cannabis mainly to sleep better or to take the edge off, this trial is a reason to check whether it is actually doing that for you, since the monitors here did not find a change. The things that reliably move sleep, glucose, and daily movement are the ordinary ones, and the returns show up in small changes to sleep, exercise, and diet linked to years of added life.