Improving Needed Safeguards for Users of Lifesaving Insulin Now Act of 2023 or the INSULIN Act of 2023 This bill limits cost-sharing for insulin under private health insurance and revises the requirements for approval and cost-sharing of generic drugs and biosimilar products. Specifically, the bill caps cost-sharing under private health insurance for a month's supply of selected insulin products at $35 or 25% of a plan's negotiated price (after any price concessions), whichever is less, beginning in 2025. The bill also requires pharmacy benefit managers (PBMs) to remit to the group health plan 100% of the fees, alternative discounts, and all other remuneration received from a drug manufacturer. Additionally, the bill modifies the Food and Drug Administration's (FDA) procedures for determining whether a petition to delay the approval of a generic drug was made with the primary purpose of delaying the application and provides for the referral of such petitions to the Federal Trade Commission. The bill also authorizes the FDA to expedite the development and review of applications for products designated as a competitive biosimilar therapy. Finally, the bill permits prescription drug plan sponsors under Medicare Part D to make certain mid-year changes to the cost-sharing tier of a reference biological product if the sponsor also adds the respective biosimilar product to the same or a lower cost-sharing tier as the reference biological product immediately prior to the change.