Regular cannabis users had 9% fewer extra heartbeats on days they were randomly assigned to inhale

Nobody in this trial was new to cannabis. Cardiologists at the University of California, San Francisco, recruited 108 adults who smoked or vaped cannabis on a regular basis and had no history of heart rhythm problems. For 14 days, the team randomly assigned each day to cannabis or abstinence and sent the verdict by text message.

On the days randomized to inhaled cannabis, the volunteers’ hearts produced about 9% fewer extra beats than on days without it, Adi Elias, Gregory Marcus and colleagues report in the Journal of the American College of Cardiology. The team had expected the opposite. “We were surprised by the results because they contradicted our original hypothesis,” said Marcus, a cardiologist at the university.

Until now, assumptions about cannabis and the heart were drawn largely from observational studies, which follow users without assigning anyone to take the drug. The trial is among the first to test the short-term effects of inhaled cannabis on the heart in people living their normal daily lives. Its authors warn that the results should not be interpreted as evidence that cannabis benefits heart health.

Premature atrial contractions are extra heartbeats that track with atrial fibrillation

When the signal to beat comes too early, it’s called a premature or extra heartbeat, and it may feel like your heart skipped a beat. Doctors call these ectopic beats, and Britain’s National Health Service says heart palpitations of this kind are common and not usually a sign of anything serious.

Extra heartbeats that begin in the heart’s upper chambers are known as premature atrial contractions, while those originating in the lower chambers are called premature ventricular contractions. Cardiologists watch the upper-chamber kind partly because of atrial fibrillation, which causes an irregular and fast heartbeat and is the most common type of arrhythmia, or heart rhythm problem. In the Cardiovascular Health Study, adults with more premature atrial contractions on a 24-hour heart recording were more likely to develop that condition later. The researchers behind that analysis noted that it does not establish a causal link.

Cannabis was already a suspect. Marcus and colleagues had previously combed the records of more than 23 million adult Californians who received care in an emergency department, outpatient surgery facility or hospital. Cannabis use noted in those files was associated with increased incidence of atrial fibrillation, meaning more new cases: about 35% more after accounting for other factors. So when the team registered its trial, the question it posed was “Does cannabis use increase the frequency of ‘early’ and abnormal heart beats?”

How a daily text message turned each cannabis user into their own comparison

Answering that from records runs into a stubborn problem, because cannabis users can also differ from nonusers in drinking, smoking, stress and much else. The San Francisco team compared each person with themselves instead, in a randomized crossover trial to assess the effects of inhaled cannabis. Each volunteer lived through both kinds of day and served as their own control.

A random draw decided which days were cannabis days, so nobody could save their cannabis for, say, an evening that was already stressful. Any difference between the two kinds of day can therefore be credited to the assignment, which lets the trial speak to cause for this group over these two weeks.

To qualify, adults had to have inhaled cannabis in the past month, and on four or more days in a single week sometime in the past year. They used their own supply and were asked to smoke or vape cannabis at least once on days they were told to use it. Anyone with a history of atrial fibrillation or heart failure, or with a pacemaker, was excluded.

Everyone wore a patch that recorded their heart rhythm continuously, along with an activity tracker, sleep monitor and continuous glucose monitor. They tapped a button on the patch at the time of cannabis use, and the researchers checked compliance using those time stamps, daily surveys and, in some of the volunteers, lab analysis of saliva. Not everyone did as the texts said, and the authors describe adherence as imperfect but generally supportive of the assignments. In all, the trial logged 713 days randomized to cannabis and 616 days randomized to abstinence.

Cannabis days brought fewer atrial extra beats but no change in sleep, steps or blood sugar

The drop in extra beats across those days was driven by fewer premature atrial contractions, which fell by about 10%, while premature ventricular contractions showed no clear change. The abstract does not report how many extra beats the volunteers started with, so there is no telling how many actual beats the drop represents.

Each cannabis inhalation was associated with about 2% fewer extra beats. A dose pattern like that usually strengthens the case for a real effect. Here it is weaker evidence than the main comparison, because chance decided which days were cannabis days while the users decided how often to inhale.

Step counts, sleep duration and glucose levels, meaning blood sugar, showed no clear differences between cannabis days and abstinence days. The method has picked up changes before. When Marcus’s team used it to test caffeinated coffee, coffee days brought more daily premature ventricular contractions, more steps and fewer minutes of nightly sleep.

Nobody yet knows why extra beats fell on cannabis days. The authors write that “potential withdrawal effects, moderate adherence, and co-intervention limit interpretation of causal mechanism.” Co-intervention means something else changing along with the assignment, such as drinking more coffee or alcohol on days without cannabis, and both are common causes of heart palpitations. The abstract does not say whether anything like that happened.

The first item on the authors’ list could change what the result means. Anyone who stops or sharply cuts back heavy or long-term cannabis use can get withdrawal symptoms, including feeling nervous or anxious, restlessness and insomnia. If a day without cannabis unsettled some regular users, extra beats could have risen on abstinence days, and the comparison would look just the same as if cannabis had lowered them. The schedule never asked anyone to abstain for more than two consecutive days, though.

Marcus sees a lead in the result: “these findings suggest there may be something about cannabis that can teach us more about the biological mechanisms that produce premature atrial contractions,” he said.

First-time users, older adults and long-term heart risk sit outside the cannabis trial

The 108 participants (mean age 32) were young regular users. Their results say nothing about someone trying cannabis for the first time, and little about older adults or anyone with heart disease. “This study examined the acute effects of cannabis in people who use it regularly and was not designed to evaluate the long-term health consequences,” Marcus said. “The overall health effects of inhaled cannabis may still be net negative.”

According to the National Institute on Drug Abuse, cannabis can increase heart rate and blood pressure right after use, and some research shows an association between long-term use and an increased risk of stroke, heart attack and arrhythmias. It says more work is needed to determine if there is a direct connection or if other factors are involved. An American Heart Association scientific statement concluded that cannabis may have therapeutic benefits, but few are cardiovascular in nature.

This article draws on the paper’s abstract, because the full text is paywalled. Details such as how much cannabis each person used, and the authors’ own account of the study’s limits, could not be checked.

For anyone who notices skipped or extra beats, the NHS advises seeing a doctor if palpitations keep coming back or last longer than a few minutes. Those that come with chest pain, shortness of breath or feeling faint call for emergency help. And the researchers caution that these findings should not influence medical or behavioral decisions around cannabis use.

If the result holds up, it will likely say more about what sets off extra heartbeats than about whether cannabis is safe to use.