Protecting Access to Safe and Effective Medicines Act of 2021 This bill provides statutory authority for the existing Food and Drug Administration (FDA) practice of defining active ingredient more narrowly as active moiety in certain situations, such as when determining whether a new drug is entitled to a market exclusivity period or providing priority review of drugs for treating rare pediatric diseases. Generally, the FDA defines active moiety as the core molecule or ion in a drug responsible for the relevant physiological or pharmacological action. By contrast, the FDA defines an active ingredient as a component in a drug that is intended to furnish pharmacological activity or other direct effect. The FDA's existing practice of interpreting active ingredient as active moiety in certain situations, as statutorily authorized by this bill, tends to exclude some drugs from market exclusivity. The bill replaces references to active ingredient with active moiety in various statutes authorizing FDA activities.