Habitual cannabis users experienced roughly 9% fewer premature heartbeats on days they inhaled cannabis compared with days they abstained, according to a randomised crossover trial led by the University of California, San Francisco (UCSF) and published in the Journal of the American College of Cardiology.
Study design and key findings
The trial enrolled 108 adults who regularly smoked or vaped cannabis and who had no history of clinically diagnosed cardiac arrhythmias. Over a period of 14 days, participants were randomly assigned each day to either inhale cannabis or to abstain. While living their normal daily lives, participants wore continuous physiological monitors: an electrocardiographic recorder, an activity tracker, a sleep monitor and a continuous glucose monitor. Each participant therefore served as their own control, enabling direct comparison of heart rhythm and other measures on use and non‑use days.
Researchers reported that cannabis inhalation was associated with a statistically significant reduction in extra or early heartbeats, known medically as cardiac ectopic beats. The headline result was a roughly 9% decrease in premature heartbeats on inhalation days compared with abstinence days.
Why the result surprised researchers
The study challenges assumptions drawn largely from prior observational work, which suggested cannabis could be associated with increased cardiac events. According to the reporting, Gregory Marcus, MD, MAS, an endowed professor of atrial fibrillation research in the UCSF Division of Cardiology, said:
"We were surprised by the results because they contradicted our original hypothesis. That's why randomized trials are so important. When the data challenge our expectations, our job as scientists is to understand them, not ignore them."
The investigators emphasised that their findings are preliminary and do not justify changes in clinical care or personal behaviour relating to cannabis use.
Strengths and limits of the trial
The study’s strengths include continuous, objective monitoring of heartbeats rather than reliance on participant self‑report or medical records. That permitted measurement of physiological changes in real time under everyday conditions. The crossover design — in which participants acted as their own controls — reduced confounding by fixed individual characteristics.
However, the trial focused on habitual inhalational users without known arrhythmia diagnoses. This limits how far the findings can be generalised, particularly to new cannabis users, people who use non‑inhalational formulations, or those with established cardiac disease. The authors cautioned that observational and mechanistic data on cannabis and cardiovascular risk remain complex and evolving.
- Population: 108 habitual cannabis smokers or vapers without diagnosed arrhythmias
- Duration: 14 days, with daily randomised assignments
- Main outcome: ~9% fewer premature heartbeats on inhalation days
Implications for clinicians and the public
The study adds an important experimental datapoint to the literature on cannabis and cardiac electrophysiology, but it does not settle the question of whether cannabis is harmful or beneficial to heart health. Clinicians should continue to assess cardiovascular risk using the full range of established risk factors and remain cautious when advising patients about cannabis, particularly those with known heart disease.
For members of the public, the researchers’ measured message is clear: evidence is evolving, and a single trial — even a well‑conducted one — should not be used as a green light to start or change cannabis use for perceived heart benefits.
Readers are reminded that individual health decisions should be made in consultation with a medical professional. If you have concerns about heart rhythm, palpitations, chest pain or any new symptoms, contact your clinic or doctor rather than self‑diagnosing or altering medication or substance use based on one study.
