18 Background: The use of oral anti-cancer medications- which are often high priced with significant cost sharing to the patient- is increasing. Transparency of out-of-pocket costs and payment responsibility has historically been limited due to the wide variation in coverage by insurance plans and pharmacy benefits. Our study utilized a novel pharmacy database to describe within-drug and between-drug variation in patient copayments for ten common oral anti-cancer medications. Methods: We conducted a retrospective cohort study including all patients prescribed an oral cancer-directed therapy through the University of California, San Francisco (UCSF) Specialty Pharmacy between January 1,2021 and December 31, 2021. The unit of analysis was defined as patient-drug. Copayments were standardized as 30-day costs. Data was abstracted through our institution’s Specialty Pharmacy prescribing database and linked to demographic data through the institutional electronic medical record system. Between drug variation in copayments was analyzed using Pearson chi square tests. Results: There were 1,357 total patient-drug combinations over the study period. Medicare Part D was the primary payor (44%) followed by commercial insurance (25%). The ten most frequently prescribed medications (Table) accounted for 50% of all oral anti-cancer prescriptions. Minimum monthly copayment for all medications was $0. There were significant differences across medications in the percentage of patients with a mean copayment above $150 (Χ2 = 33.9, p < 0.001) and percentage of patients receiving financial assistance (Χ2 = 63, p < 0.001). Conclusions: There was a high amount of within-drug and between-drug variation in copayments within an integrated specialty pharmacy program. It is essential to identify patients with high copayments early on in cancer treatment and connect them with tangible resources to help mitigate the growing financial impact of these medications.[Table: see text]