BACKGROUND: To analyze safety in patients who switched from a reference drug to a biosimilar and to characterize providers’ perceptions about the prescribing of biosimilars. METHODS: This retrospective chart review was conducted at a multi-site academic health system and included patients who received bevacizumab or trastuzumab and were switched to the respective biosimilars, bevacizumab-awwb or trastuzumab-dkst, between October 2019 and February 2021. A survey was also administered to hematology/oncology providers that characterized their comfort with prescribing and their knowledge of biosimilars. RESULTS: This study included 185 patients, of whom 88 were in the bevacizumab group and 97 were in the trastuzumab group. After converting to a biosimilar, 25% and 23% of the patients who received bevacizumab and trastuzumab, respectively, had new adverse events. After the conversion, there were 2 instances of hypersensitivity in the bevacizumab group and 1 in the trastuzumab group. Only 2% and 6% of patients in the bevacizumab and trastuzumab biosimilar groups, respectively, switched back to the reference drug. Of the 41 providers surveyed, the majority felt comfortable or very comfortable prescribing a biosimilar for an FDA-approved indication (68%). The most common concerns noted were insurance coverage (30%) and the level of knowledge or comfort with biosimilars (20%). CONCLUSION: The results of this study demonstrated that conversion from a reference drug to a biosimilar is safe. Although the majority of providers felt comfortable prescribing biosimilars, insurance coverage and level of knowledge about or comfort with prescribing biosimilars were common concerns. [ABSTRACT FROM AUTHOR]