Sports leagues, state legislators, and universities are changing the way they treat and punish marijuana use largely because the drug is increasingly perceived as harmless. But preliminary research suggests cannabis may have a dangerous side effect on one critical organ: the heart.

The new study, which evaluated the health of 3,407 people in the United Kingdom, suggests a link between regularly using marijuana — defined as daily or weekly use within the past five years — and changes to the heart’s structure and functions. It was published Wednesday in the journal JACC Cardiovascular Imaging.

Researchers observed that the study participants who used cannabis regularly had larger left ventricles and showed early signs of impaired heart function.

The left ventricle is one of the hardest working and thickest parts of the heart, pumping blood throughout the body. Changes to the physiology of this crucial machinery can lead to heart problems and ultimately, failure. Without a healthy heart, staying alive becomes difficult.

However, lead author and Queen Mary University of London researcher Mohammed Khanji tells Inverse that it’s important to remember this study is based on early data, which includes a relatively small group of regular cannabis users. The changes they detected, he explains, were subtle.

Still, he notes that, given the findings, people who recreationally use cannabis may want to “consider reducing their intake until further systematic research becomes available, which will hopefully provide further insight on the long-term effects of recreational cannabis use.”

Heart changes

Khanji and his colleagues evaluated participants without heart disease, a group that in this case was 96 percent white with an average age of 62. Their information was pulled from the UK Biobank, a database including 50,000 volunteer participants.

The participants also self-reported their cannabis habits. Most never or rarely used the substance: Out of the 3,407 study participants, only 47 were currently using marijuana regularly. Meanwhile, 105 claimed they once used the drug regularly, but that use was over five years ago. While this study evaluated recreational use, it did not include how the participants used it.

The participants had previously undergone cardiac magnetic resonance imaging (MRI) tests, which capture detailed and sensitive measures of heart health. The research team examined these MRI scans and compared the images to how often the participants used the drug.

The team's study is the first to systematically report differences in heart structure and function associated with recreational cannabis using cardiac MRI. Encyclopaedia Britannica/ Getty Images

Only the currently active regular users had larger left ventricles and showed early signs of impaired heart function, measured by how the heart muscle fibers change shape during heart contraction.

Even with these structural changes, all three groups showed the same overall mass of the left ventricle and the amount of blood ejected with each heartbeat.

People who regularly used cannabis over five years before the study did not show structural or functional cardiac changes. This finding suggests adjusting cannabis habits could reverse some of the potentially damaging effects of too much cannabis use.

Limits to the data

The study’s authors stress that the findings are not conclusive. The study has limitations — most of the participants were white, the study included a small number of regular cannabis users, and the researchers relied on participants’ self-reporting of a drug that is currently illegal in the United Kingdom.

"We urgently need systematic research to identify the long-term implications of regular consumption of cannabis.”

But although the findings aren’t definitive, they do emerge at a time when the decriminalization and legalization of recreational cannabis are ongoing. Research on any potential adverse health effects is pivotal, Khanji explains, especially because the drug is increasingly available.

Frequent, and current, marijuana use could affect the heart. PX Here

“We urgently need systematic research to identify the long-term implications of regular consumption of cannabis on the heart and blood vessels,” Khanji says. “This would allow health professionals and policymakers to improve advice to patients and the wider public.”

In turn, this study builds on decades of research on cannabis’ heart effect. Previous research suggests that cannabis intoxication causes acute tachycardia or elevated heart rate. Meanwhile, a study from 1977 suggests that middle-aged men with a history of heart attacks who smoke marijuana can also experience acute symptoms of angina, or chest pain caused by reduced blood flow to the heart. Another study, which evaluated 1,913 adults, showed that cannabis use upped the risk of cardiac-related death for patients already diagnosed with cardiovascular disease.

The research on how marijuana affects the heart is limited but ideally growing — the researchers behind this new study hope it will spark further research and debate.

Partial abstract:

Results: Analysis included 3,407 individuals (age of 62 7 years; 55% females) with 47 current regular, 105 previously regular and 3,255 rare/non-users. Current regular users were more likely to be younger, male, current tobacco smokers and with greater levels of social deprivation compared to rare/ never users and previous regular users. They were also less likely to be on antihypertensive medication. Regular cannabis use was associated with larger indexed left ventricular end-diastolic (+5.31ml/m2, 95% CI: 1.4 – 9.3 mls/m2, p=0.008), end-systolic volumes (+3.3mls/m2, 95% CI: 0.78 – 5.83 mls/m2, p=0.010), and impaired GCS (-0.78, 95% CI: -1.47 – -0.09, p=0.026) compared to rare/no cannabis use, even after adjustment for potential confounders including age, sex, body mass index, systolic blood pressure, use of cholesterol medication, diabetes, smoking and alcohol consumption (Figure 1). Following multivariable adjustment, there were no differences between for left ventricular myocardial mass, ejection fraction and stroke volume, or right ventricular, left atrial and right atrial parameters. Previous cannabis users had similar parameters to rare/no use.

Conclusions: Regular cannabis use was independently associated with adverse changes in left ventricular size and subclinical dysfunction compared to rare/no cannabis, whereas previous regular cannabis use was not. Findings should be interpreted with caution and further research is required to understand the potential pathophysiology, dose-response effects of cannabis use and the long-term implications of regular use on the cardiovascular system. Health care professionals and policy makers may need to advise caution on regular recreational cannabis use until such systematic research is available.