BACKGROUND: Oral oncolytics are becoming increasingly common in the treatment of solid and hematological malignancies. Medication adherence is especially important to ensure adequate drug levels to treat active malignancies, notably in curative-intent therapy. Further data are needed to quantify and confirm the effects of internal health-system specialty pharmacies (HSSPs) on medication adherence. OBJECTIVE: To confirm the effect of an internal HSSP compared with external specialty pharmacies on oncolytic adherence as measured by proportion of days covered (PDC), medication possession ratio (MPR), and time to treatment (TTT). METHODS: This single-center retrospective cohort study included patients receiving oral oncolytics through an internal HSSP or external specialty pharmacies between January 2019 and June 2020. Fill data were extracted from pharmacy claims databases and electronic medical records. The primary adherence outcome was patient-level PDC. Secondary adherence outcomes included patient-level MPR and TTT. For PDC and MPR analyses, patients with at least 3 fills per oncolytic were included. All patients were included for the TTT analysis. Chi-square or Fisher's exact tests were used to analyze categorical differences between pharmacy groups. Differences in continuous variables across pharmacy groups were evaluated using Wilcoxon rank-sum tests. RESULTS: 871 prescriptions met inclusion criteria: 549 patients were included in the PDC/MPR analysis, and 758 patients were included in the TTT analysis (patients might have multiple prescriptions). Patients who filled at an internal HSSP had a higher median PDC compared with those who filled at external specialty pharmacies (0.99 [IQR = 0.89-1.00] vs 0.91 [IQR = 0.76-0.98]; P < 0.01). The adherence rate as measured by MPR was higher for patients who used an internal HSSP compared with those who used external specialty pharmacies (MPR = 1.00 [IQR = 0.90-1.00] vs 0.93 [IQR = 0.76-1.00]; P < 0.01). Median TTT was lower for patients using the internal HSSP vs an external specialty pharmacy (5 days [IQR = 2-13] vs 27 days [IQR = 2-82], respectively; P < 0.01). CONCLUSIONS: Internal HSSP services improved adherence as measured by PDC and MPR. Significantly lower TTT was seen with the internal HSSP compared with external pharmacies. These data confirm and support use of internal HSSPs to dispense oral oncolytics for treatment of solid and hematological malignancies. DISCLOSURES: This study received no financial support. The authors declare no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.