Background: The approval pathway for biosimilars was established in 2010 to facilitate drug competition and generate cost savings. Biosimilar filgrastim products (filgrastim-sndz or tbo-filgrastim) were the first approved biosimilars in the US. This study examined study outcomes (incidence of febrile neutropenia (FN), growth factor related adverse events (AE) and filgrastim treatment cost) between users of biosimilar and reference filgrastim drugs among the commercially-insured US population. Methods: A retrospective observational design was used to analyze administrative claims data on adults with cancer treated with chemotherapy between 09/01/2012 - 4/30/2017. The population was grouped into biosimilar filgrastim users (B) and filgrastim users (F) according to the earliest filgrastim use (index date). FN risk factors were assessed within 6 months prior to the index date. Study outcomes were assessed within 21 days post index date using multivariable regression modelling. Results: Among 1,694 biosimilar users and 10,460 filgrastim users, no meaningful differences in baseline FN risk factors were observed. In adjusted results, no difference in incidence of FN was observed between the two groups (B: 13.3%; F: 11.0%; odds ratio [OR]: 1.17; 95% confidence intervals [CI] 0.99-1.38; p = 0.06). Rates of AE were similar between the two groups (B: 6.9%; F: 6.4%; OR: 1.04; 95% CI: 0.84-1.29, p = 0.72). The mean number of filgrastim doses administered was 3.4 in both groups (Relative difference: 0.01; 95% CI: -0.02-0.04; p = 0.53). The mean filgrastim drug costs were similar between the two groups (B: $2,491; F: $2,510, Cost Ratio [CR]: 0.96; 95% CI: 0.91-1.01; p = 0.12). However, the mean filgrastim drug cost was 6.7% less for filgrastim-sndz when compared to reference filgrastim (sndz: $2,342; CR: 0.91; 95% CI: 0.84-0.99; p = 0.04); but not for tbo ($2,549; CR: 0.97; 95% CI: 0.91-1.04; p = 0.40). Conclusions: Biosimilar filgrastim is similar to the reference filgrastim in drug safety and effectiveness. The early adoption and saving impact of biosimilar filgrastim has been modest, which highlights the structural and regulatory challenges to biosimilar uptake.