AimsDrug metabolism might be altered in patients with type 2 diabetes. We aimed to evaluate if initiation of glucose‐lowering drugs impacts warfarin efficacy and drug metabolism.MethodsFirst, we conducted a register‐based self‐controlled cohort study on Danish and Scottish warfarin users. Warfarin efficacy (international normalized ratio [INR]) was compared before and after initiation of glucose‐lowering drugs. Second, we conducted a clinical pharmacokinetic trial comprising treatment‐naïve type 2 diabetes patients. Patients ingested probe drugs for drug‐metabolizing enzymes (the Basel Cocktail) before initiating glucose‐lowering treatment, and after 3 and 12 weeks of treatment. Drug metabolism, glycaemic control, and inflammation were assessed on each visit.ResultsIn the Danish and Scottish cohorts (n = 982 and n = 44, respectively), initiating glucose‐lowering drugs reduced warfarin efficacy. INR decreased from 2.47 to 2.21 in the Danish cohort (mean difference −0.26; 95% CI −0.35; −0.17) and from 2.33 to 2.13 in the Scottish cohort (−0.21; 95% CI −0.52; 0.11) after initiation of glucose‐lowering treatment. This impact on INR was more pronounced among individuals with stronger effects of glucose‐lowering treatment. In the clinical pharmacokinetic trial (n = 10), initiating metformin did not affect drug metabolism after 3 weeks (geometric mean ratio of CYP3A metabolic ratio: 1.12 [95% CI: 0.95; 1.32]) or 12 weeks of metformin treatment. Glycaemic control improved during treatment, while inflammation remained low and unchanged during treatment.ConclusionsIn conclusion, initiation of glucose‐lowering drugs among chronic warfarin users seems associated with a reduction in INR, particularly among individuals with a large decrease in HbA_1c. This effect seems unrelated to CYP enzyme activity and warfarin drug metabolism.