Drug–drug interactions with warfarin: A systematic review and meta‐analysis

AIMS: The objective of this paper is to systematically review the literature on drug-drug interactions with warfarin, with a focus on patient-important clinical outcomes. METHODS: MEDLINE, EMBASE and the International Pharmaceutical Abstract (IPA) databases were searched from January 2004 to August 2019. We included studies describing drug-drug interactions between warfarin and other drugs. Screening and data extraction were conducted independently and in duplicate. We synthesized pooled odds ratios (OR) with 95% confidence intervals (CIs), comparing warfarin plus another medication to warfarin alone. We assessed the risk of bias at the study level and evaluated the overall certainty of evidence using the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach. RESULTS: Of 42 013 citations identified, a total of 72 studies reporting on 3 735 775 patients were considered eligible, including 11 randomized clinical trials and 61 observational studies. Increased risk of clinically relevant bleeding when added to warfarin therapy was observed for antiplatelet (AP) regimens (OR = 1.74; 95% CI 1.56-1.94), many antimicrobials (OR = 1.63; 95% CI 1.45-1.83), NSAIDs including COX-2 NSAIDs (OR = 1.83; 95% CI 1.29-2.59), SSRIs (OR = 1.62; 95% CI 1.42-1.85), mirtazapine (OR = 1.75; 95% CI 1.30-2.36), loop diuretics (OR = 1.92; 95% CI 1.29-2.86) among others. We found a protective effect of proton pump inhibitors (PPIs) against warfarin-related gastrointestinal (GI) bleeding (OR = 0.69; 95% CI 0.64-0.73). No significant effect on thromboembolic events or mortality of any drug group used with warfarin was found, including single or dual AP regimens. CONCLUSIONS: This review found low to moderate certainty evidence supporting the interaction between warfarin and a small group of medications, which result in increased bleeding risk. PPIs are associated with reduced hospitalization for upper GI bleeding for patients taking warfarin. Further studies are required to better understand drug-drug interactions leading to thromboembolic outcomes or death.