In 2012, the America Invents Act created a new administrative pathway called inter partes review to allow challenges to patents as being improperly granted, including patents associated with brand-name prescription drugs that block timely competition. Manufacturers of expensive biologic drugs often obtain large numbers of patents on drugs approved by the Food and Drug Administration, which can make it harder for other manufacturers to market their own biosimilar versions of the products. We found that biologic drugs were associated with forty-three administrative patent challenges via inter partes review during the period 2012-21 that resulted in final written decisions. Biosimilar manufacturers used this pathway in twenty cases to invalidate fourteen patents, typically directed to new methods of treatment. By contrast, biologic manufacturers selling competing brand-name products in the same drug class used this pathway to invalidate seventeen patents in the twenty-three challenges they brought, typically directed to active drug ingredients. After patent invalidation by biosimilar firms, revenues for the originator products tended to decrease, a signal of more effective competition making drugs more accessible for the benefit of patients and payers.