Can marijuana use cause bradycardia (low heart rate) in individuals, particularly those with pre-existing heart conditions such as coronary artery disease or heart failure?

Marijuana Typically Causes Tachycardia, Not Bradycardia—But Rare Cases of Severe Bradycardia Have Been Reported

Marijuana predominantly causes tachycardia (increased heart rate) through sympathetic nervous system stimulation, but paradoxically can cause bradycardia in rare cases, particularly with cannabidiol (CBD) use or chronic heavy consumption. This is critical for patients with pre-existing coronary artery disease or heart failure, as both tachycardia and bradycardia can precipitate acute cardiac events 1, 2.

Primary Cardiovascular Effect: Tachycardia

The overwhelming evidence demonstrates that marijuana's typical cardiovascular response is tachycardia, not bradycardia:

• Marijuana causes dose-dependent increases in heart rate and blood pressure by blocking catecholamine reuptake at presynaptic terminals, directly stimulating the sympathetic nervous system 2
• This tachycardic effect is accompanied by coronary vasoconstriction, increased endothelin-1 levels, decreased nitric oxide, and a prothrombotic state through platelet activation 2
• The American Society of Clinical Oncology explicitly lists tachycardia among the "more serious side effects" requiring patient counseling 2
• Combined tobacco and marijuana use produces greater increases in heart rate than either substance alone 1, 2

Rare But Documented: Marijuana-Induced Bradycardia

Despite tachycardia being the norm, bradycardia can occur and may be life-threatening:

Case Evidence of Severe Bradycardia

• A 51-year-old woman with chronic marijuana use developed complete heart block requiring permanent pacemaker placement—the first reported case of complete heart block from chronic marijuana use 3
• A 39-year-old man experienced four syncopal episodes with extreme bradycardia and a 13.8-second sinus pause after inhaling CBD, demonstrating that cannabinoids' action on adenosine receptors can induce life-threatening bradycardia 4
• A systematic review identified four cases (10%) of symptomatic bradycardia among 42 case reports, along with third-degree atrioventricular block (12%) and asystole (12%) 5

Mechanism of Bradycardia

• CBD specifically acts on adenosine (anti-arrhythmic) receptors, which can trigger severe bradycardia and sinus pauses 4
• Orthostatic hypotension occurs acutely from decreased vascular resistance, which may be accompanied by bradycardia 1, 6

Clinical Implications for High-Risk Patients

For patients with coronary artery disease or heart failure, both the typical tachycardic response and rare bradycardic episodes pose significant risks:

Tachycardia Risks

• Decreased exercise test duration and increased heart rate at submaximal exercise levels can precipitate myocardial ischemia 6
• One study demonstrated marijuana may trigger myocardial infarction onset 6
• Patients with angina pectoris experience decreased time until chest pain onset 6

Bradycardia and Conduction System Risks

• Advanced conduction system abnormalities including complete heart block can occur with chronic use 3
• Symptomatic bradycardia with syncope represents a medical emergency requiring immediate evaluation 4

Practical Recommendations

Counsel patients with coronary artery disease or heart failure to avoid marijuana entirely due to unpredictable cardiovascular effects that can trigger acute cardiac events 1, 6:

• The predominant risk is tachycardia with coronary vasoconstriction, which can precipitate ischemia or infarction 2, 6
• However, rare cases of severe bradycardia, complete heart block, and prolonged sinus pauses have been documented 3, 4, 5
• CBD products carry particular risk for bradycardia through adenosine receptor activation 4

For patients presenting with unexplained syncope, bradycardia, or new-onset heart block, specifically inquire about acute and chronic cannabis use, including CBD products 4, 5.

Common Pitfall

Do not assume marijuana only causes tachycardia. While this is the typical response, the rare but documented cases of severe bradycardia and complete heart block mean clinicians must maintain vigilance for both extremes of heart rate abnormalities in marijuana users 3, 4, 5.