A general equation for the ratio of the areas below the blood or plasma concentration time-curves following intravenous and oral drug administration and its application to inter-subject variations in drug elimination.

1 When a drug is metabolized in the liver the total area under the plasma or blood concentration-time curve following oral drug administration is less than the area obtained after intravenous drug administration. This difference has been termed the 'first-pass effect'. 2 General equations for the areas below the blood or plasma concentration-time curve of a drug following intravenous and oral drug administration are derived. 3 An explicit expression for the ratio of areas below the blood or plasma concentration-time curves following intravenous and oral drug administration is derived. 4 By the application of the derived equations it is shown that the relative contribution of hepatic drug metabolism and urinary excretion to inter-subject variation in drug elimination can be easily ascertained.