BACKGROUND: In 2023, Washington state Medicaid eliminated prior authorizations (PAs) for antiretrovirals (ARVs). We estimated the budget impact of this policy change on Medicaid ARV expenditures, 2023-2027. METHODS: Models used 2022 net drug costs reflecting costs minus rebates. Our base case pre-exposure prophylaxis (PrEP) model assumed changes in the proportions of PrEP users on tenofovir disoproxil fumarate/emtricitabine (TDF/FTC), tenofovir alafenamide/emtricitabine (TAF/FTC), and cabotegravir (CAB) based on 2021-2022 data (prior to elimination of PAs). We compared base case costs to models assuming changes in PrEP prescribing observed 2022-2023 (first year after elimination of PAs), and models assuming an accelerated increase in TAF/FTC and CAB use. For HIV treatment, models assumed ARV changes based on 2021-2022 data (Base Model), changes based on data from 2022-2023 (No PA - Stable Increase), and a model based on 2022-2023 data but with a declining rate of change in the adoption of bictegravir/TAF/FTC and CAB/rilpivirine (No PA - Declining Increase). We estimated the number of persons with HIV (PWH) who might be housed using money required to meet new ARV costs using 2023 local Ryan White Program costs for emergency or temporary housing. RESULTS: Elimination of PAs will increase Medicaid expenditures for ARVs by an estimated 09.9-57.7 million over 5 years. This cost would pay for 5 years of housing for approximately 820-1,177 people, or 61-88% of unstably housed PWH in Washington state. CONCLUSIONS: Elimination of Medicaid PAs will result in substantial new costs. Changes in drug formulary policy should consider opportunity costs.