Objective: To describe amitriptyline treatment modifications during a supply-restriction period and evaluate changes in pain intensity in an exploratory analysis. Methods: This single-center retrospective study included adults prescribed amitriptyline for pain management between January and December 2022 who remained on treatment when prescribing restrictions began in April 2023. Discontinuation or switching was assessed through March 2024. The primary endpoint was the proportion of patients who discontinued amitriptyline or switched to another drug for any reason. Treatment modifications were classified as either documented shortage-related or attributable to other clinical reasons. Numerical Rating Scale (NRS) changes were explored among shortage-related cases with paired baseline and 8-week assessments. Results: Of 180 patients screened, 83 were included. During the observation period, 81 patients (97.6%) underwent treatment modification for any reason, and 52 of 83 (62.7%) had treatment modifications explicitly documented as shortage-related. The remaining 29 modifications were attributed to inadequate pain control and/or adverse events. Among the 52 shortage-related cases, paired NRS scores were available for 33 patients. Median NRS scores were 5 [interquartile range (IQR), 3–7] at baseline and 5 [IQR, 4–6] at 8 weeks, with no significant overall change (p = 0.21). Changes did not differ significantly between the discontinuation and switching groups (p = 0.48). All patients with paired NRS scores were treated in the Department of Anesthesiology. Conclusions: Documented shortage-related treatment modification occurred in approximately two-thirds of eligible patients. The NRS findings were exploratory and should be interpreted cautiously because of potential selection bias.