Twenty-five states have implemented insulin out-of-pocket (OOP) cost caps, but their effectiveness is uncertain.

To examine the effect of state insulin OOP caps on insulin use and OOP costs among commercially insured persons with diabetes.

Pre–post study with control group.

Eight states implementing insulin OOP caps of $25 to $30, $50, or $100 in January 2021, and 17 control states.

Commercially insured persons with diabetes and insulin users younger than 65 years. Subgroups of particular interest included members from states with insulin OOP caps of $25 to $30, enrollees with health savings accounts (HSAs) that require high insulin OOP payments, and lower-income members.

Mean monthly 30-day insulin fills and OOP costs.

State insulin caps were not associated with changes in insulin use in the overall population (relative change in fills per month, 1.8% [95% CI, −3.2% to 6.9%]). Insulin users in intervention states saw a 17.4% (CI, −23.9% to −10.9%) relative reduction in insulin OOP costs, largely driven by reductions among HSA enrollees; there was no difference in OOP costs among nonaccount plan members. More generous ($25 to $30) state insulin OOP caps were associated with insulin OOP cost reductions of 40.0% (CI, −62.5% to −17.6%), again primarily driven by a larger reduction in the subgroup with HSA plans.

Single national insurer; 9-month follow-up.

Insulin OOP caps were associated with reduced insulin OOP costs but no overall increases in insulin use. A proposed national insulin cap of $35 for commercially insured persons might lead to meaningful insulin OOP savings but have a limited effect on insulin use.