Erdafitinib (ERDF), despite its known adverse effects, remains a key chemotherapeutic agent for metastatic urothelial carcinoma, requiring reliable monitoring in biological matrices to ensure safe and effective treatment. In this study, we developed a cost-effective, energy-efficient method for synthesizing highly fluorescent nitrogen-doped carbon dots (NCDs) using simple precursors. The resulting NCDs displayed excellent photostability and a high fluorescence quantum yield of 41.87%. ERDF induced a concentration-dependent quenching of the NCDs' fluorescence at 520 nm, attributed to the inner filter effect, with a low detection limit of 0.013 μM (S/N = 3). The NCDs demonstrated high selectivity toward ERDF with minimal interference from coexisting substances. Recovery rates in spiked human serum and urine ranged from 97.8% to 104.4%, validating the method's practical applicability. The relative standard deviations (RSDs) remained below 3.72%, confirming the precision and reproducibility of the NCD-based sensing platform.