Background: Insurance plans vary coverage for infusional (IV) vs oral drugs, leading some to suggest that patients on oral drugs pay more OOP than those on IV drugs. 43 states have passed laws requiring insurers to cover oral drugs equivalently to IV drugs. Yet, there is little evidence that these “parity laws” are effective. Our aim was to estimate impact of parity laws on OOP expenses for oral vs IV drugs. Methods: We sought to determine how quickly patients on oral vs IV drugs reach their plan’s annual OOP maximum (max) as a surrogate for OOP expense. We used 2017 data from Healthcare.gov public use files to generate cost-sharing profiles for all 3,092 unique Marketplace plans. Chronic lymphocytic leukemia (CLL) and metastatic hormone sensitive prostate cancer (mHSPC) were chosen as two representative malignancies since both have accepted, first-line, IV and oral treatment options. We created guideline-concordant, first-line treatment regimens for simulated patients with CLL (oral ibrutinib vs IV bendamustine/rituximab) or mHSPC (oral abiraterone vs IV docetaxel). Drug, professional, facility, imaging, and lab claims were simulated to calculate OOP costs. The mean number of days to reach the OOP maximum for each Marketplace plan and treatment regimen were recorded. We assessed variation according to insurance coverage levels (“metal tier”: Catastrophic, Bronze, Silver, Gold, Platinum). Results: For CLL patients, 95% of plans reached OOP max in approximately one month of treatment for both oral and IV drugs (oral: mean 36 days; IV: mean 29 days). 99% of mHSPC patients reached their OOP max for oral treatment in a mean 15 days, but only 57% of plans reached OOP max for IV mHSPC treatment. Metal tier impacts time to reach OOP max (table). Conclusions: Parity laws do not lower patient costs when both IV and oral treatment options are expensive. In these cases, patients reach the OOP max rapidly. The small subset of patients most likely to benefit from parity laws are those on oral therapy for a disease where the comparable IV drug is inexpensive (eg, generic docetaxel for mHSPC).