A new randomized clinical trial is challenging some assumptions about the immediate cardiovascular effects of cannabis. Researchers led by the University of California, San Francisco found that habitual cannabis users experienced approximately 9 percent fewer premature heartbeats on days when they inhaled cannabis than on days when they abstained. The study, titled Acute Effects of Cannabis Inhalation on Cardiac Ectopy, Physical Activity, Sleep, and Glucose, appears in the Journal of the American College of Cardiology and was highlighted on August 12, 2026. It represents an unusual addition to cannabis research because investigators experimentally assigned cannabis-use and abstinence days rather than simply comparing people who use cannabis with people who do not.
The result sounds at first like evidence that cannabis might protect the heart from abnormal rhythms, but that conclusion would go considerably beyond what the trial actually demonstrated. The reduction was primarily in premature atrial contractions, or PACs—early electrical beats originating in the heart’s upper chambers. Premature ventricular contractions, or PVCs, did not significantly change, and investigators did not measure whether cannabis prevented atrial fibrillation, stroke, heart attack, heart failure or cardiovascular death. The researchers instead view the unexpected result as a clue about how medical cannabis use may acutely influence the electrical and autonomic processes that generate certain premature beats.
How the MARY-JANE Trial Worked
The study, called MARY-JANE—short for Marijuana and Acute Risk of Arrhythmia-Joint Abstinence and Exposure—enrolled 108 adults who regularly smoked or vaped cannabis but had no diagnosed cardiac arrhythmia. Their average age was 32, 59 percent were women and 37 percent identified as non-Hispanic White. Rather than placing participants in a laboratory and administering a standardized dose, investigators used a pragmatic 14-day crossover design intended to capture cannabis use in ordinary life. Participants received daily randomized instructions telling them either to inhale cannabis or abstain that day, meaning the same person contributed both cannabis-use and cannabis-free observations and effectively served as their own control.
Throughout the study, participants wore continuous electrocardiographic monitors capable of recording every heartbeat. Researchers also collected activity, sleep and continuous glucose data. Compliance was evaluated through daily surveys, time-stamped button presses on the ECG device when cannabis was consumed and, in a subset of participants, saliva testing using mass spectrometry. Across the trial, investigators accumulated 713 participant-days assigned to cannabis inhalation and 616 assigned to abstinence. This within-person design helps control for many characteristics—such as genetics, age and underlying physiology—that complicate conventional observational cannabis studies.
Cannabis Use Was Associated With 9 Percent Less Cardiac Ectopy
The primary cardiovascular outcome was cardiac ectopy, meaning premature beats occurring outside the heart’s normal rhythm. When all premature atrial and ventricular beats were combined, cannabis-use days were associated with a 9 percent reduction in ectopic beats, corresponding to a rate ratio of 0.91. The 95 percent confidence interval ranged from 0.85 to 0.98, and the result reached statistical significance with a P value of 0.02. Investigators had expected cannabis might increase ectopy, making the direction of the result particularly surprising.
Looking more closely revealed that the effect came almost entirely from the atria. Premature atrial contractions fell approximately 10 percent on cannabis-use days, with a rate ratio of 0.90 and a 95 percent confidence interval from 0.82 to 0.97. PVCs originating in the ventricles did not significantly change. Researchers also observed an apparent exposure-response relationship: each additional episode of cannabis inhalation was associated with approximately a 2 percent reduction in total ectopic beats. That association is intriguing, but because participants chose their own cannabis products and dosing patterns, it cannot establish which cannabinoid—or even cannabis itself rather than a behavioral consequence of use—produced the effect.
Why Premature Atrial Contractions Matter
PACs are extremely common and often harmless, particularly when they occur occasionally in otherwise healthy people. Their importance comes from research showing that a greater burden of atrial ectopy can predict future atrial fibrillation. In a prospective cohort study of 1,260 adults without atrial fibrillation, UCSF researchers and collaborators found that every doubling of the hourly PAC count was associated with a 17 percent higher risk of subsequently developing atrial fibrillation. PAC burden improved long-term prediction beyond conventional clinical risk factors, although the study could not show that suppressing PACs itself would prevent atrial fibrillation.
That distinction is essential when interpreting the new cannabis trial. A 10 percent reduction in PACs over individual cannabis-use days does not mean a 10 percent reduction in atrial fibrillation risk. Nor does it demonstrate that deliberately using cannabis to suppress PACs would provide a clinical benefit. Participants were relatively young habitual users without known arrhythmias and generally had a low burden of ectopic beats. To establish a cardiovascular benefit, researchers would need considerably larger and longer trials showing that reducing PACs through a particular intervention actually lowers rates of sustained arrhythmia, stroke, hospitalization or other meaningful outcomes.
The Finding Runs Against Expectations About Cannabis and the Heart
One reason the study attracted attention is that cannabis is already known to cause measurable acute cardiovascular changes. Controlled research has found that THC-predominant cannabis can increase heart rate and blood pressure after either smoking or vaping. Another human physiology study found that cannabis inhalation increased heart rate and mean arterial pressure while reducing muscle sympathetic nerve activity. These apparently contradictory responses illustrate that cannabinoids can affect several interacting systems simultaneously, including cannabinoid receptors, blood vessels and autonomic nervous system activity.
That complexity could help explain why atrial ectopy moved in an unexpected direction. One possibility raised by the physiology is that changes in autonomic balance influence the electrical behavior of atrial tissue even while heart rate increases. Cannabis can also produce relaxation in habitual users, alter circulating catecholamines and change vascular tone. But the JACC trial was not designed to identify a mechanism, and its results do not demonstrate that THC itself is responsible. Participants used their usual smoked or vaporized cannabis rather than a standardized THC, CBD or terpene formulation, making it impossible to isolate individual chemical components.
Sleep, Activity and Glucose Did Not Explain the Difference
Researchers monitored several other factors because cannabis could theoretically alter heart rhythm indirectly. If people became less physically active while using cannabis, slept substantially longer or experienced meaningful changes in glucose, any of those effects could potentially influence autonomic tone and cardiac ectopy. Yet the investigators found no statistically significant differences between cannabis-use and abstinence days in daily step counts, sleep duration or average glucose levels.
Those negative results make certain simple explanations less likely, but they do not reveal what caused the reduction in PACs. The study also cannot completely separate a beneficial effect of cannabis-use days from a possible adverse effect of short-term abstinence. Participants were habitual users, and repeated abstinence days could have produced subtle withdrawal-related physiological changes in some people. Cannabis withdrawal can include sleep disruption, anxiety, irritability and autonomic symptoms, raising the possibility that at least part of the apparent difference resulted from what happened when regular users stopped rather than what happened when they consumed cannabis. The JACC investigators explicitly identify potential withdrawal and imperfect adherence as important limitations.
The Result Does Not Overturn Research Linking Cannabis to Cardiovascular Risk
The finding also needs to be separated from the broader question of long-term cardiovascular health. One of the largest recent observational analyses examined data from more than 430,000 U.S. adults and found that more frequent cannabis use was associated with higher odds of cardiovascular events. Compared with non-use, daily cannabis use was associated with 25 percent higher adjusted odds of myocardial infarction and 42 percent higher odds of stroke. Similar associations remained in analyses involving people who had never smoked conventional cigarettes, although the observational design cannot prove that cannabis caused those events.
Earlier research has also raised concerns about acute cardiovascular events. A well-known 2001 Circulation study investigating triggers of myocardial infarction reported that the risk of heart attack onset was temporarily elevated during the first hour after marijuana use. More recent experimental work has documented vascular and hemodynamic changes after cannabis exposure, while the American Heart Association has emphasized that cannabis can influence heart rate, blood pressure, vascular function and drug metabolism. None of this is incompatible with the new PAC result: an exposure can reduce one physiological marker while simultaneously affecting other cardiovascular pathways in less favorable ways.
Why the Randomized Design Is Still Important
Much of the debate over cannabis and cardiovascular disease has relied on observational studies, which are vulnerable to confounding. Cannabis users can differ from non-users in tobacco exposure, alcohol consumption, socioeconomic circumstances, diet, prescription drug use and numerous other characteristics. People may also inaccurately report their cannabis consumption, and commercially available products vary enormously in THC concentration and chemical composition. A crossover trial cannot solve every problem, but randomizing individual cannabis-use days provides a powerful way to investigate immediate physiological responses while reducing differences between participants.
At the same time, the pragmatic nature of MARY-JANE introduces its own uncertainty. Participants did not receive a uniform cannabis strain or standardized THC dose, assignment adherence was imperfect, and investigators could not blind participants to whether they were using cannabis. The study involved people already accustomed to inhaled cannabis, so the findings may be very different in cannabis-naive individuals. It also excluded people with known arrhythmias—the exact population in whom a change in ectopic beats might have the greatest clinical importance. Results therefore cannot be extrapolated to patients with atrial fibrillation, ventricular arrhythmias, heart failure or significant cardiovascular disease.
A Physiological Clue Rather Than Evidence Cannabis Protects the Heart
Perhaps the most scientifically useful interpretation of the study is that cannabis may reveal something unexpected about the mechanisms controlling atrial ectopy. Researchers originally hypothesized that inhalation would increase premature beats. Instead, randomization produced the opposite result for PACs while leaving PVCs essentially unchanged. That separation between the heart’s upper and lower chambers suggests cannabis exposure—or something accompanying cannabis use or withdrawal—may selectively influence atrial electrophysiology or autonomic regulation rather than simply making the entire heart more or less electrically excitable.
Future research could use standardized THC and CBD preparations, defined doses, blood cannabinoid measurements and controlled abstinence periods to determine which component is responsible. Studies involving older adults and patients with high PAC burdens or established arrhythmias would also help determine whether the effect is reproducible and clinically meaningful. Until that happens, the new JACC trial should not be interpreted as a reason to begin or increase cannabis use for heart health. Instead, it demonstrates why randomized cannabis research remains essential: even a drug with documented cardiovascular concerns can produce acute physiological effects that run directly against expectations, and understanding those contradictions may ultimately teach researchers more about both cannabis and the electrical behavior of the human heart.
