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.
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