Medications Contraindicated with Medical Cannabis
Medical cannabis is not absolutely contraindicated with any specific medication class, but significant drug-drug interactions exist that require careful monitoring, particularly with anticoagulants, antiplatelet agents, and medications metabolized by CYP2C9, CYP2C19, and CYP1A2. 1
Key Drug Interactions Requiring Caution
Anticoagulants and Antiplatelet Agents
Warfarin requires the most vigilance when combined with cannabis. Cannabis, particularly CBD, inhibits CYP2C9, which metabolizes warfarin, potentially leading to increased plasma concentrations, elevated INR, and bleeding risk. 2 Multiple case reports document this interaction resulting in supratherapeutic anticoagulation. 2
Clopidogrel efficacy may be compromised because CBD inhibits CYP2C19, the enzyme responsible for converting clopidogrel to its active metabolite. 2, 3 This could result in subtherapeutic antiplatelet effects and increased stroke risk in high-risk cardiovascular patients. 2
Medications Metabolized by Cytochrome P450 Enzymes
Cannabis constituents (THC, CBD, and CBN) affect multiple CYP enzymes with varying clinical significance: 3
- CYP2C9, CYP1A1/2, and CYP1B1: Likely inhibited by all three major cannabinoids (THC, CBD, CBN) at physiologically achievable concentrations 3
- CYP2D6, CYP2C19, CYP2B6, and CYP2J2: Inhibited by THC and CBD 3
- CYP3A4/5/7: Potentially inhibited by CBD 3
- CYP1A2: Induced through cannabis smoking 3
Clinical evidence confirms CYP2C19 inhibition by CBD and CYP2C9 inhibition by various cannabis products. 3
Other Enzyme Systems
- UGT1A9: Inhibited by CBD and CBN 3
- UGT2B7: Inhibited by CBD but activated by CBN 3
- Carboxylesterase 1 (CES1): Potentially inhibited by THC and CBD, though clinical data remain limited 3
Perioperative Analgesic Medications: No Contraindications
Cannabis use does NOT contraindicate the following perioperative analgesics: 1
- NSAIDs
- Opioids
- Intravenous or regional local anesthetics
- Ketamine
- Gabapentin or pregabalin
- Dexmedetomidine
- Acetaminophen/paracetamol
While in vitro studies suggest cannabinoids inhibit COX enzymes and animal studies show reduced NSAID response with chronic THC use, no clear evidence of clinically significant antagonism, synergy, or additive effects exists in humans. 1
Populations Where Cannabis Should Be Avoided
Cannabis use should be avoided entirely in the following high-risk populations: 4, 5
- Pregnant and breastfeeding individuals: Risk of fetal brain development impairment and premature birth 6
- Patients with psychotic spectrum disorders: Risk of worsening psychotic symptoms 5
- Patients with bipolar disorder: Risk of worsening mania symptoms and function 5
- Adolescents and young adults: 2-fold to 11-fold increased risk of psychosis, higher risk of cannabis use disorder, and increased self-harm in those with mood disorders 5
- Patients with ischemic heart disease: Daily cannabis use associated with increased risk of coronary heart disease (2.0% vs 0.9%), myocardial infarction (1.7% vs 1.3%), and stroke (2.6% vs 1.0%) 4
Clinical Monitoring Recommendations
For patients using cannabis with potentially interacting medications: 4
- Avoid concurrent use with alcohol or other CNS depressants (especially benzodiazepines)
- Use the lowest effective cannabis dose
- Avoid use when driving or operating machinery
- Monitor closely for signs of drug toxicity or reduced efficacy
- Consider checking drug levels (e.g., INR for warfarin) more frequently
Special Perioperative Considerations
For patients consuming high amounts of cannabis (>1.5 g/day smoked, >300 mg/day CBD oil, or >20 mg/day THC oil): 1
- Consider additional PONV prophylaxis
- Anticipate increased postoperative analgesic requirements
- Give extra consideration to processed EEG monitoring for anesthetic depth
- Maintain vigilance for cannabis withdrawal syndrome (begins 48 hours after abstinence)
Cannabis withdrawal symptoms include irritability, insomnia, headaches, nausea, and stomach pain. 6, 1