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. The fraction of drug reaching the systemic circulation is expressed as the bioavailability. The concept of bioavailability is important in practice because the clinician can use routes of administration that maximize bioavailability. Bioavailability is the fraction of a substance, whether a drug, nutrient, or supplement, that actually reaches your bloodstream in its active form after you take it. The meaning of BIOAVAILABILITY is the degree and rate at which a substance (such as a drug) is absorbed into a living system or is made available at the site of physiological activity. 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. In pharmacology, bioavailability is a subcategory of absorption and is the fraction (%) of an administered drug that reaches the systemic circulation. [1] 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. 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 simply means the fraction of administered drug that reached the systemic circulation (blood). It can range from 0% (no drug) to 100% (all of the administered drug). The adjectives “absolute” and “relative” are commonly added to the bioavailability term. Bioavailability is essential in the development of new drugs and formulations. More effective and efficient active ingredients can be designed by understanding the factors that affect bioavailability, such as solubility, chemical stability, and pharmaceutical forms.