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Background and Importance Pembrolizumab and nivolumab are monoclonal antibodies whose initial dosing strategies were based on bodyweight, 2 mg/kg every 3 weeks and 3 mg/kg every 2 weeks, respectively. However, fixed dosing (FD), 200 mg every 3 weeks and 240 mg every 2 weeks, was subsequently introduced after showing equivalence.Aim and Objectives To describe the cost saving of switching from pembrolizumab and nivolumab FD to weight-based dosing (WBD), as recommended by hospital management, in real-world clinical practice.Material and Methods A retrospective observational study was conducted in a tertiary hospital, including patients who received at least one dose of pembrolizumab or nivolumab between September 2024 and August 2025. Collected data (extracted Farmatools ) from F included patient weight, dosage regimen and number of administrations. FD and WBD followed the official product label. Cost analysis considered vial prices (pembrolizumab 1081.75€; nivolumab 641.73€), quantities used, and vial optimisation. The cost of WBD was compared with a hypothetical FD scenario, estimating the percentage of patients receiving WBD and potential savings. Differences between groups were assessed using an independent samples Student’s t-test.Results Pembrolizumab was administered to 287 patients: 223 (weight 69.50 ± 28.22kg) received fixed doses and 64 (weight 62.61 ± 14.37kg) received weight-based doses. Annual cost was 3,684,439.82€ for fixed dosing and 454,925.32€ for weight-based dosing, saving 267,683.32€ (6.47%). Weight-based dosing was used in 22.3% of patients. No significant difference in mean bodyweight was found between groups.Nivolumab was administered to 108 patients: 64 (weight 62.29 ± 32.06kg) received fixed doses and 44 (weight 64.95 ± 14.79kg) received weight-based doses. Annual cost was 1,058,857.64€ with fixed dosing and 297,505.63€ with weight-based dosing, saving 244,629.48€ (18.04%). Weight-based dosing was applied in 40.74% of patients. Although a significant difference in mean bodyweight was found (t=2.81, p < 0.05), the absolute difference was small and likely clinically irrelevant.Conclusion and Relevance Individualised dosing of nivolumab and pembrolizumab can represent a strategy to optimise treatment costs in clinical practice. However, in our institution, the proportion of patients receiving WBD remains suboptimal. Continued education, data sharing, and consensus protocols with prescribers are needed to promote WBD for these monoclonal antibodies, optimising treatment outcomes and reducing healthcare costs.Conflict of Interest No conflict of interest