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 a substance or drug becomes completely available to its intended biological destination (s). In pharmacology, bioavailability is a subcategory of absorption and is the fraction (%) of an administered drug that reaches the systemic circulation. [1] 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 amount of a nutrient in your food that actually gets absorbed and used by your body. It’s not the same as the total amount listed on a nutrition label. 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 is referred to as the extent and rate to which the active drug ingredient or active moiety from the drug product is absorbed and becomes available at the site of drug action. 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. In practice, bioavailability indicates the amount of the administered dose of a drug reaching the bloodstream in the form of the active ingredient which is then available to the body to produce a therapeutic effect [2]. 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.