Effect of THC on arrhythmia

The evidence retrieved does not directly test whether THC causes or worsens heart arrhythmias in humans, so the burden of proof for safety has not been met.

None of the 31 records studied THC's effect on cardiac rhythm in people. The closest studies look at other drugs (ajmaline, TCAs, carbon monoxide) or at cannabis's effects on the brain and lungs. One dog study [1] documents THC poisoning causing tremors and muscle spasms but does not report heart rhythm. The CB1 receptor study [3] is about brain myelination, not the heart. A large UK Biobank analysis [2] gives background rates of arrhythmia in the general population (about 5 in 1000 people per year develop a new rhythm problem), but it does not separate cannabis users from non-users.

The mechanism for harm is plausible. THC activates CB1 receptors on heart tissue and blood vessels, which can raise heart rate and lower blood pressure—both triggers for arrhythmia in susceptible people. But no human trial or observational study in this retrieval measured whether that actually happens. The question of whether THC is safe for someone with existing arrhythmia, or whether it can cause new arrhythmia in a healthy person, is unanswered by the research surfaced here. A study that compared arrhythmia rates in matched cannabis users versus non-users over time would settle it, and no such study is present.

My call: the available evidence does not show whether THC affects arrhythmia risk, because it was not designed to answer that question. The absence is not safety. Confidence: low.