Affordable Prescriptions for Patients Act of 2021 This bill prohibits product hopping by drug manufacturers, authorizes the Federal Trade Commission to enforce this prohibition, and imposes limits on patent litigation involving biological products. Generally, product-hopping describes a situation where, when the patents on a reference drug (or biological product) expire, the manufacturer switches to a follow-on product that is covered by a later-expiring patent. Under this bill, a follow-on product is a modified version of the reference drug that shares an indication (what the drug is used for) with the reference drug. The bill presumes product hopping has occurred when a reference drug manufacturer, after receiving notice that the Food and Drug Administration (FDA) has received an application to market a competing generic (or biosimilar) version, takes certain actions such as withdrawing the reference drug from the market and selling a follow-on product. A drug manufacturer may rebut these presumptions by demonstrating that its conduct was not intended to limit competition. The bill also limits in certain instances the number of patents that a reference biological product manufacturer can assert in a patent infringement lawsuit against a company seeking to sell a biosimilar version. Specifically, if the biosimilar manufacturer completes certain actions as part of an abbreviated pathway to get FDA market approval, the bill limits, subject to exceptions and waivers, the number of certain types of patents that the reference product manufacturer may assert, such as patents filed more than four years after the reference product received market approval.