How Safe Is Cannabis for the Heart?

This is a summarized version of the full in-depth article on Relias Media.

By Michael H. Crawford, MD. Professor of Medicine, Lucy Stern Chair in Cardiology, University of California, San Francisco.

Study Overview

The safety of cannabis use, especially for medical purposes, is an area of growing concern due to its widespread prescription for chronic pain and its potential cardiovascular effects. While recreational cannabis use has been associated with adverse cardiac events, including tachycardia, arrhythmias, and acute coronary syndromes (ACS), little is known about the safety of its medical use, particularly regarding new-onset arrhythmias. In a recent study by Holt et al., Danish researchers aimed to assess the relationship between medical cannabis use and the risk of developing new arrhythmias in patients with chronic pain.

This study utilized nationwide health registry data from Denmark, where a trial prescribing cannabis products (containing cannabidiol [CBD] and tetrahydrocannabinol [THC]) for chronic pain was conducted from 2018 to 2021. Patients with a history of cannabis use or pre-existing arrhythmias were excluded. Each patient prescribed cannabis was matched with five control patients, all diagnosed with chronic pain but not using cannabis, ensuring parity in age, sex, diagnosis, and concurrent pain medication use.

Study Population and Methods

• Patients: The study population included almost 2 million individuals diagnosed with chronic pain between 2018 and 2021, of which 5,391 patients (mean age 59 years, 63% women) were treated with medical cannabis. The cannabis types used were: 47% THC, 24% CBD, and 29% a combination of both.

• Controls: 26,941 patients with chronic pain, matched on key characteristics but not using cannabis, served as controls.

• Follow-Up: Patients were followed for 180 days or until the study’s end in 2021. The primary outcome was the occurrence of new-onset arrhythmias or conduction disorders, while secondary outcomes included ACS, stroke, syncope, ectopy, and heart failure.

Key Findings

During the 180-day follow-up period, new-onset arrhythmias were identified in 42 cannabis users and 107 control patients. The risk of developing arrhythmias was significantly higher in cannabis patients compared to controls (risk ratio [RR] 2.07; 95% confidence interval [CI] 1.34-2.80). While the absolute incidence of arrhythmias was relatively low (0.77% in cannabis users), the findings suggest a two-fold increased risk of arrhythmias in those using medical cannabis for chronic pain.

• Types of Arrhythmias: Among the cannabis users who developed arrhythmias, 79% had atrial fibrillation/flutter, 14% experienced conduction disorders, and 7% developed other types of arrhythmias.

• Secondary Outcomes: Importantly, there was no significant increase in the risk of other major cardiovascular events, including ACS (RR 1.2; 95% CI 0.35-2.04), stroke (RR 0.99), or heart failure (RR 0.63), in the cannabis group compared to controls.

Certain subgroups, particularly patients with cancer or cardiometabolic diseases, exhibited the highest risk of arrhythmias, highlighting the potential vulnerability of these populations to the adverse cardiovascular effects of medical cannabis.

Commentary and Clinical Implications

The study's findings contribute to the evolving understanding of the cardiovascular safety of medical cannabis, especially given its increasing use for chronic pain management. While the absolute risk of arrhythmias was small, the doubling of the risk among cannabis users underscores the need for cautious use, particularly in patients with pre-existing risk factors such as cancer or cardiometabolic conditions.

The cardiovascular effects of cannabis are believed to be due to its stimulation of the sympathetic nervous system and inhibition of the parasympathetic system, leading to increased heart rate and changes in cardiac conduction. These effects are well-documented in recreational users, with reports of almost every acute cardiovascular event, including arrhythmias, ACS, and stroke. The fact that these effects are also observed in medical cannabis users, albeit to a lesser extent, raises concerns about its safety profile in vulnerable populations.

Study Limitations

While this study provides valuable insights, several limitations must be acknowledged:

1. Adherence and Dosing: Not all patients refilled their initial cannabis prescriptions, making it unclear whether the arrhythmias observed were related to transient, early effects of cannabis use or continued exposure. Additionally, the specific doses of THC and CBD used by patients were not known, which could affect the generalizability of the results.
2. Route of Administration: The method of cannabis delivery (inhalation, oral spray, oral solution, or pills) was not consistently documented, and different routes may have varying effects on cardiovascular risk.
3. Comparison to Recreational Cannabis: The doses of active ingredients in medical cannabis products are likely lower than those found in recreational products, particularly synthetic cannabinoids, which are known to be more potent. Therefore, the cardiovascular risks associated with recreational use may be higher than those observed in this study.
4. Limited Generalizability: The study population was relatively young (mean age 59 years) with a low incidence of comorbidities, which may limit the applicability of these findings to older, more medically complex populations. This large, nationwide study from Denmark suggests that medical cannabis use for chronic pain is associated with an increased risk of new-onset arrhythmias, particularly atrial fibrillation/flutter. However, the overall incidence of arrhythmias was low, and no significant increase in other major cardiovascular events, such as ACS or stroke, was observed. 

The study highlights the need for caution when prescribing medical cannabis, particularly in patients with cancer or cardiometabolic conditions, who may be at higher risk for cardiovascular complications. Clinicians and patients should be aware of these potential risks, and further research is needed to fully understand the long-term safety of medical cannabis, especially in comparison to recreational use and in different patient populations.

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We discuss the safety of cannabis for the heart in more detail and include detailed charts and tables in our full write-up on Relias Media.

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