Bioavailability

 Definition 
 * Bioavailability (F) is a pharmacokinetic parameter representing the fraction of a drug dose that reaches the systemic circulation unaltered.

 Characteristics 
 * The greater the F, the greater the amount of drug that reaches the systemic circulation, and thus the greater the drug concentration in plasma and vice versa
 * The route with the highest F (F=1) is via intravenous (I.V.) administration.
 * The second highest F is achieved through inhalation.
 * The parenteral routes of drug administration have low F because all the drug absorbed by the GIT, passes through the hepatic circulation undergoing first pass metabolism, prior entering to the systemic circulation.

 Types 

Absolute bioavailability
 * It is the comparison of the per os F to the intravenous F
 * $$ {F_{abs}} = {{AUC_{P.O.} \times D_{I.V.}\over AUC_{I.V.}\times D_{P.O.}}\times 100 } $$
 * where
 * AUCP.O.: area under the curve for the oral route
 * AUCI.V.: area under the curve for the intravenous route
 * DP.O.: dose administered from oral route
 * DI.V.: dose administers from intravenous route

Relative bioavailability
 * It is the comparison of the per os F to another administration route, other than intravenous.