• A new study finds that people who take medical cannabis have twice the risk of atrial fibrillation (AFib), a type of heart arrhythmia, although the risk is still low.
  • AFib can lead to heart attacks, strokes, and blood clots.
  • Recreational cannabis has also been linked to an increased risk of heart attack.

According to a new study from Denmark, people who have been prescribed medical cannabis are at a slightly higher risk of developing atrial fibrillation, particularly within the first 180 days.

The study found that patients taking medical cannabis were 0.8% — eight-tenths of 1% — more likely to develop new-onset atrial fibrillation in their first six months of cannabis treatment.

Meanwhile, the patients not taking medical cannabis saw a 0.4% increased risk of new-onset AFib after 180 days.

In the U.S., as of April 2023, 38 states, the District of Columbia, and three territories currently allow medical cannabis. Several European countries, including the U.K., Portugal, Spain, and the Netherlands, as well as other countries around the world, are now allowing its legal medicinal use, with more expected to follow.

Atrial fibrillation, or AFib, is an irregular, or arrhythmic heartbeat, that has been linked to more serious cardiovascular outcomes such as stroke, heart failure, and blood clots if left untreated.

The study is an analysis of data from 5,391 Danes using medical cannabis for relief from chronic pain, compared with people who were not using cannabis. The participants’ median age was 59, and 63.2% were women.

For both groups, the likelihood of developing AFib at six months was less than 1%.

In the new study, the largest increase in AFib was observed in patients with cardiometabolic disease and cancer.

The study is published in the European Heart Journal.

Although the first six months of medical cannabis use were associated with a greater difference in AFib risk, after a year, the difference had lessened somewhat. Lead author Dr. Anders Holt from the Department of Cardiology at Copenhagen University Hospital suggested some possible explanations.

“Perhaps,” he said, “any side-effects present rather acutely making the patient stop treatment during follow-up, thus diluting the differences at longer follow-up.” In addition, he said, “It could be due to patients halting treatment during follow-up for other unknown reasons.”

Dr. Holt also mentioned that there may be other confounding factors that make the overall picture less clear.

Dr. Holt said that his study’s finding is primarily remarkable considering the lack of research into the effects of medical cannabis.

“However, the observational nature of the study design and the very modest risk differences found do not merit a major health concern, in my opinion,” he said.

Dr. Holt tells the European Heart Journal in a press release that he doesn’t think this study should deter patients with chronic pain “from trying medical cannabis if other treatment has been inadequate.”

Dr. Holt told Medical News Today the results should simply “serve as a reminder that all medical treatment may carry the risk of side effects and that we should always make sure that benefits outweigh possible risk.”

On the other hand, Dr. Cheng-Han Chen, cardiologist and medical director of the Structural Heart Program at the MemorialCare Heart & Vascular Institute, who was not involved in the study, found the study’s conclusion alarming.

“Even though the absolute risk is quite low, it’s a doubling in the risk,” said Dr. Chen.

Also concerning to him is that these are younger people who really shouldn’t be getting atrial fibrillation in his opinion.

Cardiologist Dr. Jayne Morgan, who was also not involved in the study, described what medical cannabis patients may want to look out for:

“Symptoms of atrial fibrillation independent of any cause would be palpitations, shortness of breath, fatigue, dizziness, rapid or irregular heartbeat.”

To that list, Dr. Chen added lightheadedness and fatigue.

Though clinical research regarding cannabis has been scant due to its long-standing illegality in many places, there have been more studies investigating recreational cannabis than medical cannabis.

“To my knowledge,” said Dr. Holt, “this is the first, larger, nationwide cohort study investigating cardiovascular side effects related to prescribed medical cannabis.”

“These studies are new, and more of them are needed,” said Dr. Morgan.

Dr. Holt also called for more controlled studies of medical cannabis.

Recreational cannabis, said Dr. Chen, has been “tied to increased risk of heart attack, especially in younger folks.”

Among the current unknowns are cannabis’s health effects on older people. “A more advanced age with known cardiovascular disease is a category more commonly encountered with cardiology patients and would need to be given consideration,” pointed out Dr. Morgan.

Recreational cannabis has also “been associated with elevated risk of arrhythmia and acute coronary syndromes,” said Dr. Holt.

An interesting note is that although the new study saw a link between medical cannabis and AFib, it did not detect one with acute coronary disease.

Among the difficult-to-track variables with medical cannabis, said Dr. Chen, is that “the medical model is not the same as when we as physicians prescribe a medicine for a heart condition, in the sense that we’re always monitoring the patients very closely for side-effects and effects.”

Dosing of medical cannabis is different from putting a patient on a regular schedule of pills or capsules with known quantities.

Dr. Chen said he is not very conversant with the prescribing of medical cannabis. However, he said, “My understanding of cannabis is a lot of patients are able to get a prescription somehow from someone and just be able to get [it] filled somewhere. And I don’t think the monitoring is the same currently in the United States as if someone was getting prescribed a [typical] medication.”

Dr. Morgan cited a need to know more about short- and long-term side effects, as well as administration and dosing. Medical cannabis may be ingested as a pill, smoked, vaped, eaten, or sprayed, for example.

“When we do research,” said Dr. Chen, “we try to compare apples to apples as much as possible, and if the dosage is ‘as needed’ and how you administer the medication is widely varied, that just makes patterns much harder to detect.”

“Seeking first to do no harm, and as medical cannabis use increases, this is an area that will demand more rigor and data for physician prescribing guidance, including whether there is reversible arterial vasospasm, endothelial inflammation, and possibly vasospasm,” said Dr. Morgan.

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