Formulary Restrictions and Relapse Episodes in Persons With Relapsing-Remitting Multiple Sclerosis.

Importance: Narrow formularies can be used to increase rebates and manage the use of costly drug therapies in the US. Objective: To examine the association between the breadth of formulary coverage for multiple sclerosis (MS) disease-modifying therapies (DMTs) and MS relapse. Design, Setting, and Participants: This retrospective cohort study analyzed 100% Medicare administrative data from 2018 to 2022. The data analysis was conducted from August 1, 2024, to January 30, 2025. Beneficiaries with stand-alone prescription drug plans (PDPs) and Medicare Advantage Prescription Drug plans (MA-PDs) were followed for at least 5 quarters (baseline, quarters 1-4; follow-up, quarter 5). Calendar periods reflected formulary decision-making. Beneficiaries in the same Medicare Part D plan during baseline and follow-up, with relapsing-remitting MS, and with MS DMT use during baseline were included. Exposures: Formulary breadth was low coverage if the 4-quarter moving average of MS DMT drug or class coverage was below the median by plan type and quarter; it was considered high coverage if the average was above the median. Main Outcome and Measures: The primary outcome was MS relapse, including inpatient or outpatient treatment for MS. Multivariable logistic regressions were estimated separately for PDPs and MA-PDs, controlling for patient and plan characteristics and clustering for repeated observations by beneficiary. Results: The claims analysis included 50 162 unique beneficiaries in PDPs (mean [SD] age, 58.5 [12.1] years; 74.9% female) and 34 708 in MA-PDs (mean [SD] age, 58.2 [10.3] years; 77.2% female). Oral or injectable MS DMTs were frequently excluded from coverage (>50% excluded in 2022: PDPs, 11 of 15 DMTs; MA-PD, 9 of 15 DMTs). The MS relapse rate was greater for low- vs high-coverage PDPs (10.6% vs 9.5%; odds ratio [OR], 0.88 [95% CI, 0.84-0.92]) and MA-PDs (7.8% vs 6.9; OR, 0.88 [95% CI, 0.85-0.91]). In multivariable analyses, broader formulary coverage during baseline was associated with less MS relapse during follow-up (PDP: adjusted OR, 0.93 [95% CI, 0.90-0.96] for drugs and 0.94 [95% CI, 0.91-0.97] for classes; MA-PD: adjusted OR, 0.88 [95% CI, 0.83-0.94] for drugs and 0.92 [95% CI, 0.86-0.98] for classes). Conclusions and Relevance: In this cohort study of Medicare data, broader formulary coverage was associated with an 8% to 12% lower odds of MS relapse in MA-PDs and a 6% to 9% lower odds in PDPs. Formulary coverage and restrictions should be tailored for patient need, not just management of costs and use.