Initiation of glucose‐lowering treatment decreases international normalized ratio levels among users of vitamin K antagonists: a self‐controlled register study

Essentials It is not known if initiation of glucose-lowering drugs alters the efficacy of vitamin K antagonists (VKA). We examined if glucose-lowering drugs affected international normalized ratio (INR) in VKA-treated patients. Upon initiating glucose-lowering drugs, 51% of patients had INR values below the therapeutic window. Monitoring of INR levels should be intensified upon initiation of glucose-lowering drugs. SummaryBackground It is not known whether initiation of antidiabetic treatment affects the effect of vitamin K antagonists (VKAs). It was previously shown that metformin affects the effect of one VKA, phenprocoumon. Objectives The aim of this study was to determine if initiation of glucose-lowering treatment affects the international normalized ratio (INR) and dose requirements of the anticoagulant VKAs warfarin and phenprocoumon. Patients/methods We performed a self-controlled retrospective register-based study. A total of 118 patients commencing glucose-lowering treatment while being treated with warfarin or phenprocoumon were included in the study. We compared INR, dose/INR and proportion of patients with at least one sub-therapeutic INR measurement before and after initiation of glucose-lowering treatment. Results Initiation of glucose-lowering treatment caused mean INR to decrease from 2.5 to 2.2 (decrease of −0.3 [95% CI: −0.1; −0.5]) and led to more than half of the patients having at least one sub-therapeutic INR measurement. Six to 12 weeks later, the VKA dose/INR was increased by 11%, indicating a weakened effect of the VKA. Conclusion Initiation of glucose-lowering treatment reduces the anticoagulant effect of VKAs to an extent that is likely to be clinically relevant. This finding needs confirmation and mechanistic explanation.