This bill amends Title XVIII of the Social Security Act to modify how coinsurance for prescription drugs is calculated under Medicare Part D. For plan years beginning on or after January 1, 2026, the legislation mandates that coinsurance amounts for covered Part D drugs must be based on the drug's actual acquisition cost , rather than its wholesale acquisition cost, provided the actual acquisition cost is lower. This change applies to drug costs incurred after the annual deductible but before reaching the annual out-of-pocket threshold, specifically for drugs subject to coinsurance rather than a copayment. The "actual acquisition cost" is defined as the negotiated price of the drug under the plan, after subtracting any manufacturer-provided price concessions, as reported in the Detailed DIR Report. By linking coinsurance to the net price paid by the plan, the bill aims to ensure that beneficiaries' out-of-pocket expenses more accurately reflect the true cost of the medication. This change is intended to lower drug costs for seniors enrolled in Medicare Part D plans.