Absolute Bioavailability Vs. Relative Bioavailability

Absolute Bioavailability


When the systemic availability of an orally administered drug is calculated in comparison to its intravenous administration, it is called as absolute bioavailability.


Absolute bioavailability can be calculated by comparing the total amount of intact drug that reaches the systemic circulation following non- intravenous administration (e.g., oral, rectal, etc.) with the total amount that reaches the systemic circulation following intravenous administration.


Hence, absolute bioavailability of a drug is the systematic availability of a drug after extra vascular administration compared to intravenous administration. There are different methods available to determine the extent of systemic availability. One of the most commonly used methods is comparing AUC (area under the plasma drug concentration–time curve) after an intravenous and an extra vascular administration.




Extra vascular administration of the drug includes routes such as oral, rectal, nasal, subcutaneous, etc. Intravenous dose is used as a standard/reference to compare the systemic availability of the drug administered via different routes. When a drug is administered intravenously, the administered dose will directly enter into the systemic circulation and will face no absorption barriers to cross and hence, it is considered to be totally (100%) bioavailable. Absolute bioavailability is expressed as percentage.


Relative Bioavailability


When the systemic availability of a drug after administration is compared with that of standard of the same drug, it is known as relative bioavailability.


Relative bioavailability can be calculated by comparing the plasma drug concentration-time-curves (AUC - area under the curve) after the administration of two different formulations of the same compound (e.g. capsule vs. tablet).


Thus, relative bioavailability is the systematic availability of the drug from a dosage form as compared to the reference standard given by the same route of administration.


When a drug which cannot be administered through intravenous route, then instead of absolute bioavailability, the relative bioavailability can be calculated. In this scenario, the bioavailability of a given drug from a test dosage form is compared to that of the same drug administered in a standard dosage form (standard can be clinically proven preparation). Relative bioavailability is expressed as percentage.


Comparative study of Absolute Bioavailability and Relative Bioavailability