Ensuring Access to Lower-Cost Medicines for Seniors Act of 2021 This bill establishes additional requirements for prescription drug plan (PDP) sponsors that use formularies under the Medicare prescription drug benefit. Specifically, the bill requires PDP formularies to include covered generic drugs and biosimilars for which the wholesale acquisition cost is less than that of the reference (i.e., brand-name) product. PDP sponsors must also establish specific cost-sharing tiers that apply lower cost-sharing requirements for such covered generic drugs and biosimilars as compared to those for brand-name products. The bill also prohibits PDP sponsors from instituting certain requirements relating to access to such covered generic drugs and biosimilars that are more restrictive than those for brand-name products (e.g., prior authorization requirements).