Research suggests link between medical cannabis for chronic pain and abnormal heart rhythm

People taking medical cannabis for chronic pain have a slightly increased risk of arrhythmia, according to research published in the European Heart Journal today (Thursday). Arrhythmia is when the heart beats too slowly, too quickly or irregularly. It includes conditions like atrial fibrillation.

Recreational use of cannabis has been linked to cardiovascular disease but there has been very little research on the side effects of medical cannabis.

Researchers say the new study is important as a growing number of countries now permit medical cannabis as a treatment for chronic pain.

The study was led by Dr. Anders Holt from Copenhagen University Hospital—Herlev and Gentofte in Denmark. It included data on 5,391 Danish patients who had been prescribed cannabis for chronic pain. This included people with pain in their muscles, joints or bones, people with cancer and those suffering with nerve pain. Researchers compared this group with 26,941 patients who also had chronic pain but were not receiving cannabis as a treatment.

The data showed that patients receiving medical cannabis had a 0.8% risk of being diagnosed with arrhythmia that required monitoring and possible treatment within 180 days of receiving cannabis. This risk was more than twice the risk for patients with chronic pain who were not taking cannabis. The difference in risk between the two groups had become smaller when researchers looked at the first year of treatment.

Patients taking cannabis who were aged 60 and older and those already diagnosed with cancer or cardiometabolic disease, such as heart disease, stroke and diabetes, had the largest increases in their risk of arrhythmia.

The study did not show any link between taking medical cannabis and the risk of acute coronary syndrome, which includes a heart attack and unstable angina, stroke or heart failure.

Dr. Holt said, "Medical cannabis is now allowed as a treatment for chronic pain in 38 US states as well as several countries in Europe—such as Spain, Portugal, the Netherlands and the UK—and elsewhere around the world. This means more and more doctors will find themselves prescribing cannabis, despite a lack of evidence on its side effects.

"I don't think this research should make patients with chronic pain refrain from trying medical cannabis if other treatment has been inadequate. However, these results do suggest some improved monitoring may be advisable initially, especially in patients who are already at increased risk of cardiovascular disease."

Researchers say this is the first nationwide study of its kind investigating the cardiovascular effects of medical cannabis for chronic pain. However, they caution that this is an observational study.

Dr. Holt explained, "Despite our best efforts to make a balanced comparison, it can never be assumed that patients prescribed medical cannabis do not differ from patients not prescribed medical cannabis, and this could influence the results."

He added, "We need much more research in this area. Before concluding anything, results from this study should be replicated in other countries and settings. It would also be interesting to understand if there are any links between long-term cannabis use and heart failure, stroke, or acute coronary syndrome. This would be an important area to clarify since chronic pain can persist for many years."

In an accompanying editorial, Prof. Robert L. Page from the University of Colorado, U.S., said, "Close pharmacovigilance of cannabis, as well as its safety and efficacy, have been limited by decades of worldwide illegality and by the ongoing classification of cannabis as a Schedule 1 controlled substance in the U.S.. Nonetheless, with increased cannabis decriminalization and legalization across the globe, the association between cannabis exposure and incident cardiovascular events has emerged as an important safety signal.

"Therapeutically, these findings suggest that medical cannabis may not be a 'one-size-fits-all' therapeutic option for certain medical conditions and should be contextualized based on patient comorbidities and potential vulnerability to side effects.

"Cannabis is typically referred to as recreational and 'medical' or 'medicinal.' This latter terminology has come under scrutiny as it includes both phytochemical-derived cannabis products and also those prescription cannabinoids approved in the EU. Additionally, the term 'medical' implies that the product may have some clinical monitoring along with safety and efficacy data, which is far from true with cannabis. To this end, I would argue that 'cannabis for therapeutic use' would be a more appropriate terminology rather than 'medical.'"