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 (gt;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.
Blaylock et al. (2025) studied this question.