When a patient needs to be treated over the long term, medications need to be repeatedly administered in order to keep the concentrations within the desired therapeutic range. Maintaining a patient on an intravenous infusion is not practical, so the drug is administered intermittently (and often orally). Clinicians therefore need to consider
the rate of absorption of the product and the rate of elimination (clearance) in order to determine the dose and the frequency of drug administration.
See how dosing variations give different concentration-time curves here (module 2 for oral dosing).