Probable Interaction Between Warfarin and Inhaled and Oral Administration of Cannabis

Objective: To report a probable interaction between warfarin and edible cannabis that resulted in an elevated international normalized ratio (INR) without bleeding complications. Case Summary: A 35-year-old Middle Eastern male on warfarin long term with an INR goal of 2.5 (accepted range: 2.0-3.0). The patient has generally been stable on warfarin 10 mg daily from 2010 to 2018, until INR suddenly increased to 7.2 following 1 month of edible cannabis ingestion and cannabis smoking. Patient denied any signs and symptoms of bleeding. No other reasonable causes of the elevation in INR were apparent. The patient was advised to hold 2 doses of warfarin and discontinue cannabis use. The INR dropped below 4 upon discontinuation of cannabis with dose adjustments to warfarin. Discussion: The elevation in INR can be explained by the inhibition of CYP2C9 by cannabis use causing decreased metabolism of warfarin. The interaction between warfarin and cannabis was determined to be probable using the Horn Drug Interaction Probability Scale. Conclusions: There are no previous reports of interactions between edible cannabis and warfarin, with very few case reports describing the interaction with other forms of cannabis. Close monitoring of INR in patients with concomitant cannabis is recommended for proper warfarin management.