Bioavailability and bioequivalence of drugs: the basic concepts

Abstract

Bioavailability (BA) and bioequivalence (BE) are foundational concepts in pharmaceutical sciences, critical to
ensuring the safety, efficacy, and quality of drug products. Bioavailability refers to the rate and extent to which an
active pharmaceutical ingredient (API) becomes available at the site of action, influencing drug performance and
therapeutic outcomes. Bioequivalence, meanwhile, signifies the lack of a significant difference in bioavailability
between two pharmaceutically equivalent or alternative products when administered at the same molar dose under
similar conditions. These parameters are especially pivotal in the development and regulatory approval of generic
drugs, where demonstrating bioequivalence to a reference (innovator) product ensures therapeutic consistency and
public trust. This review synthesizes current knowledge on the principles, methodologies, and regulatory standards
related to BA and BE. A comprehensive literature search was conducted across major scientific databases (PubMed,
Scopus, Web of Science, and Google Scholar) for studies and regulatory documents published between 2000 and
2024. Search words were “bioavailability,” “bioequivalence,” “pharmacokinetics,” “generic drugs,” “drug
absorption,” “FDA guidance,” and “regulatory standards. Relevant data were extracted using defined inclusion and
exclusion criteria and analyzed through qualitative synthesis. Key pharmacokinetic concepts including absolute and
relative bioavailability are discussed, with a focus on their measurement using the area under the concentration-time
curve (AUC). The review also outlines regulatory requirements set forth by agencies such as the Food and Drug
Administration of the United States (FDA) and European medicines Agency (EMA), emphasizing methodological
considerations and challenges in BA/BE studies. Ultimately, the evaluation of BA and BE is essential for informed
drug development, regulatory decision-making, and the advancement of cost-effective, high-quality generic therapies
in modern healthcare.