BackgroundTo explore differences in outcomes between dose-adjusted vitamin K antagonists (VKAs) "new starters" and "switchers" in patients with nonvalvular atrial fibrillation (AF). MethodsA post hoc analysis was performed to assess the outcome differences between VKA "new starters" and "switchers" in AF patients using pooled individual patient data of AMADEUS and BOREALIS trials. ResultsA total of 4169 AF patients were included in the present analysis, which included 1383 "VKA new starters" and 2786 "VKA switchers". VKA new starters had higher crude rates of all-cause mortality (P = .035) and cardiovascular death (P = .047) compared to switchers. On multivariable Cox regression analysis, both "new starters" and "switchers" showed nonsignificant trends for different risks of stroke/systemic thromboembolism (SE) (hazard ratio (HR): 1.66, 95%CI: 0.95-2.90, P = .08), major bleeding (HR: 1.25, 95% CI: 0.73-2.16, P = .42), and all-cause death (HR: 1.09, 95% CI: 0.75-1.57, P = .65). On Kaplan-Meier analysis, both groups had similar risks of stroke/systemic embolism (P = .09), major bleeding (P = .28), and all-cause death (P = .06). ConclusionsIn this post hoc analysis of clinical trial patients with AF, "new starters" and "switchers" for VKA initiation had nonstatistically significant rates of trial-adjudicated thromboembolism, major bleeding, and all-cause mortality.