Background Marijuana usage is increasing world over for both medicinal and recreational purposes. Several states are legalizing cannabis and physicians can expect to encounter more patients who use or abuse marijuana. Adverse cardiovascular effects of marijuana like myocardial infarction, cardiomyopathy and arrhythmias have been well described but bradyarrythmia are rare and mechanisms are not well pronounced. Case summary A 26 years old young medical intern with history of chronic marijuana usage presented with complains of dizziness and recurrent syncope. Heart rate at presentation was 42 beats per minute and rest of the physical examination was unremarkable. There was high grade atrioventricular block in the Electrocardiogram (ECG) and subsequent electrophysiological (EP) study showed high grade supra-hisian (nodal)atrioventricular block with mildly prolonged his-ventricular (HV) interval. Urinary screen was positive for Δ9-tetrahydrocannabinol (THC). After ruling out other possible causes, diagnosis of high-grade AV block due chronic marijuana use was made. Dual chamber pacemaker was implanted, and patient was discharged in stable condition. The heart rhythm completely improved at three-month follow-up. Discussion We report a novel finding in marijuana induced bradyarrythmia. Reversible high grade atrioventricular (AV) block with electrophysiologic determination of site of conduction blockade is not reported previously. The mechanism of bradyarrythmia is thought to be mediated by increased vagal tone. However direct toxic effects of Δ9-tetrahydrocannabinol (THC), through cannabinoid receptors 1(CB1R ) on the cardiac conduction system cannot be ruled out.