Cannabis prescribed for chronic pain is associated with an elevated risk of heart rhythm disorders, according to research presented at ESC Congress 2022.

Study author Dr. Nina Nouhravesh of Gentofte University Hospital, Denmark said: “Chronic pain is a rising problem. According to Danish health authorities, 29% of Danish adults over 16 years of age reported chronic pain in 2017, up from 19% in 2000. Medical cannabis was approved in January 2018 on a trial basis in Denmark, meaning that physicians can prescribe it for chronic pain if all other measures, including opioids, have proven insufficient. Safety data are sparse, hence this study investigated the cardiovascular side effects of medical cannabis, and arrhythmias in particular, since heart rhythm disorders have previously been found in users of recreational cannabis.”

Medical cannabis comes in various formulations depending on tetrahydrocannabinol (THC) and cannabidiol (CBD) levels. Dronabinol (high THC), cannabinoid (more THC than CBD), and cannabidiol (high CBD) can be prescribed in Denmark. The drug can be inhaled, eaten, or sprayed in the mouth.

The researchers identified a total of 1.6 million patients diagnosed with chronic pain in Denmark between 2018 and 2021. Of those, 4,931 patients (0.31%) claimed at least one prescription of cannabis (dronabinol 29%, cannabinoids 46%, cannabidiol 25%). Each user was matched by age, sex and pain diagnosis to five non-users with chronic pain who acted as controls. Users and controls were followed for 180 days and their risks of new cardiovascular conditions were compared.

The median age of participants was 60 years and 63% were women. The study reports, for the first time, the chronic pain conditions of medical cannabis users in Denmark. Some 17.8% had cancer, 17.1% arthritis, 14.9% back pain, 9.8% neurological diseases,  4.4% headaches, 3.0% complicated fractures, and 33.1% other diagnoses (mostly unspecified chronic pain).

The absolute risk of new-onset arrhythmia was 0.86% in medical cannabis users compared with 0.49% in non-users, for a relative risk of 1.74. The risks of new-onset acute coronary syndrome and heart failure did not differ between the two groups. The results were similar for each chronic pain condition and each type of medical cannabis.

Dr. Nouhravesh said: “Our study found that medical cannabis users had a 74% higher risk of heart rhythm disorders compared with non-users; however, the absolute risk difference was modest. It should be noted that a higher proportion of those in the cannabis group were taking other pain medications, namely non-steroidal anti-inflammatory drugs (NSAIDs), opioids and anti-epileptics, and we cannot rule out that this might explain the greater likelihood of arrhythmias.”

She concluded: “Since medical cannabis is a relatively new drug for a large market of patients with chronic pain, it is important to investigate and report serious side effects. This study indicates that there may be a previously unreported risk of arrhythmias following medical cannabis use. Even though the absolute risk difference is small, both patients and physicians should have as much information as possible when weighing up the pros and cons of any treatment.”