Δ-9-tetrahydrocannabinol dose increase leads to warfarin drug interaction and elevated INR.

Objective: We have reported a case of a drug-drug interaction (DDI) involving warfarin and Δ-9-tetrahydrocannabinol (THC) that resulted in a supratherapeutic international normalized ratio (INR) level. The purpose of this case report is to highlight the possibility of a pharmacokinetic DDI between THC and warfarin.

Case Summary: A 67-year-old Caucasian man suffering from chronic pain presented to a dispensary in Buffalo, NY, for a refill of his medical cannabis (MC). The patient asked to speak with the pharmacist, and during their discussion the patient stated that he had a supratherapeutic INR level of 5.2 measured at home with a self-test device. The patient had no evidence of bleeding, and administration of warfarin was held for 2 days before the INR level returned to a normal range. The supratherapeutic level occurred when the patient was self-titrating his dose of THC and scored an 8, or "probable," on the Naranjo Adverse Drug Effect Probability Scale.

Practice Implications: Warfarin and cannabinoids such as THC are both metabolized by cytochrome P450 (CYP) isozymes present in the liver and gastrointestinal tract. In the case described, a dose increase of 7.35 mg THC preceded an INR elevation of 5.2, but did not result in any bleeding. These observations are suggestive of a DDI involving warfarin and THC. Clinicians involved with MC should have adequate knowledge of the drugs that act as substrates, inhibitors, and inducers of CYP enzymes, including the major cannabinoids.