Background: The introduction of trastuzumab biosimilars in clinical practice plays an important role in promoting the sustainability of healthcare systems. By contrast, the switching process can be challenging to the clinics. This survey describes the switching process at a National Cancer Institute over a period of 2 years. Methods: Data regarding all trastuzumab-based regimens for breast cancer (BC) from 1 January 2019 and 31 December 2020 were extracted from both adverse drug reactions (ADRs) reporting systems and electronic systems involved in inventory management, prescribing, dispensing, and administration. Both patients under monotherapy and combination treatment regimens were included. There were no exclusion criteria. Results and Conclusions: Overall 354 patients received at least one trastuzumab-based regimen for a total of 493 lines of treatment and 5769 administrations. Biosimilar were used in 34.3% of trastuzumab-based treatments. No differences between biosimilars and reference drug have been observed in terms of ADRs. The effective cost-saving of the first 2 years is greater than EUR 800,000 and it is estimated to increase over time.