Bioavailability (BA) refers to the fraction of an administered dose that reaches systemic circulation in its active form. It describes both the rate and extent of absorption. Bioavailability refers to the degree and rate at which a substance, such as a medication or nutrient, is absorbed into the bloodstream and becomes available to its target site of action in the body. FDA may require in vivo or in vitro testing, or both, to measure the bioavailability of a drug product or establish the bioequivalence of specific drug products. This publication discusses the basic concepts of bioavailability and the factors affecting it. We also looked at various methods of assessing bioavailability, both in the laboratory and in the clinic. When discussing the oral bioavailability of drugs, basic factors such as key characteristics of the chemical substance and the dosage form should be considered. Bioavailability is the percentage of a drug that actually reaches your bloodstream and becomes available to work in your body. When you swallow a pill, not all of it makes it into circulation. Bioavailability refers to the extent and rate at which the active moiety (drug or metabolite) enters the systemic circulation, thereby accessing the site of action. Bioavailability (F) describes the fraction of a dose of drug that reaches the systemic circulation. Bioavailability of a single drug may vary significantly depending on the route of administration. In pharmacology, bioavailability is a subcategory of absorption and is the fraction (%) of an administered drug that reaches the systemic circulation. [1] Instantly calculate absolute or relative bioavailability with visual output, formula steps, and clinical interpretation. Ideal for pharmacy students and professionals.