Single oral dosing
When a drug is administered by an oral dose, the drug must first undergo absorption before entering the systemic circulation. It is still possible to determine how much drug is necessary to reach a desired serum concentration.
For example: What oral loading dose should be given to reach a target plasma level of 1,5 ng/ml? Remember that if we administer a drug via the oral route, we have to consider the bioavailability. Again, the population parameters help: F = 0.5
(F) · Dose / Vd = Cmax or Dose = (Cmax · Vd) / F
Dose = (1,5 ng/ml · 511 L) / 0,5 = 1,5 mg
Remember, that for both the intravenous and oral dosing we have calculated the serum concentration. Sometimes a certain amount of time is necessary for all the drug to distribute over all the volume. Therefore, the actual time that a drug may start working may be delayed until it reaches its site of action.