Scientists expected cannabis to increase abnormal heartbeats.
Instead, a carefully monitored study found the opposite for one common type of early heartbeat, leaving researchers with a new puzzle about how cannabis affects the cardiovascular system.
In the trial, habitual cannabis users experienced about 9% fewer premature heartbeats on days they inhaled cannabis than on days they abstained. The decrease came mainly from early beats that started in the heart’s upper chambers.
The research was led by scientists at the University of California, San Francisco and published in the Journal of the American College of Cardiology. The investigators say the result is scientifically interesting but should not be treated as an endorsement of cannabis use.
The heart normally follows a carefully timed electrical pattern. Special cells create signals that tell the upper and lower chambers when to contract so blood can move efficiently through the body.
Sometimes an electrical signal arrives too early, producing what feels like an extra or skipped beat. These premature beats are extremely common, and healthy people can experience them without having heart disease.
When an early beat starts in an upper chamber, doctors call it a premature atrial contraction, or PAC. When it begins in one of the heart’s lower chambers, it is called a premature ventricular contraction, or PVC.
Most isolated PACs are harmless, but their frequency can provide useful information. People who have large numbers of PACs are more likely to develop atrial fibrillation later, making PAC frequency an important marker for researchers studying heart rhythm problems.
Atrial fibrillation causes the upper chambers of the heart to beat in a fast and disorganized way. It can allow blood clots to form and is an important cause of stroke, particularly in older adults.
PVCs have a different origin and can also be harmless when they happen occasionally. Very frequent PVCs, however, can sometimes signal heart disease or contribute to weakening of the heart.
To investigate cannabis under real-life conditions, the researchers enrolled 108 adults who regularly smoked or vaped it. None had previously been diagnosed with a clinical heart rhythm disorder.
The study lasted 14 days. Each day, participants were randomly instructed either to use inhaled cannabis or to abstain, creating repeated comparisons within the same individuals.
This design is important because every participant acted as his or her own control. Differences in genetics, age and many long-term lifestyle factors therefore could not easily explain why heart rhythm changed between a person’s cannabis and non-cannabis days.
Participants wore an electrocardiographic monitor that recorded their heart rhythm continuously. They also wore devices that tracked activity and sleep, while continuous glucose monitors measured changes in blood sugar.
The heart recordings showed fewer premature beats on cannabis days. When researchers separated the results by type, they found a statistically significant reduction in PACs but no meaningful difference in PVCs.
That was not what the team had predicted. Senior researcher Dr. Gregory Marcus said the investigators originally expected a different result, but randomized trials are designed precisely to reveal what happens rather than confirm what scientists expect to happen.
Researchers also saw no significant differences in how many steps participants took, how long they slept or their glucose levels. These results added to the unexpected nature of the trial.
There are several reasons the findings should not be overinterpreted. All participants were already regular cannabis users, meaning they may have developed tolerance or other adaptations that would not exist in someone trying cannabis for the first time.
The study was also short. A temporary change in heart rhythm over 14 days cannot show whether years of cannabis exposure increase the chance of heart attack, stroke, heart failure or dangerous rhythm disorders.
Smoking and vaping themselves can also expose the body to chemicals and particles with possible health effects. The study did not weigh every potential benefit and harm of cannabis, so fewer PACs should not be confused with better overall cardiovascular health.
The researchers instead see the result as a clue about the biology of premature atrial contractions. Cannabis contains compounds that affect receptors throughout the nervous and cardiovascular systems, and understanding which pathway reduced PACs could reveal new information about why these early beats occur.
That knowledge might eventually help scientists develop treatments that influence the same pathway without requiring cannabis exposure. Before that can happen, researchers need larger and longer studies to confirm the result and uncover the mechanism behind it.
The study’s greatest value may be its reminder that health effects can look different when tested in randomized real-world experiments.
Cannabis remains a complicated exposure, and one unexpected short-term heart rhythm effect does not settle the much larger question of whether regular use helps or harms cardiovascular health.
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Source: University of California, San Francisco
