After a single dose administration we assume that there is no drug in the body before the drug is administration and that no more is going to be administered. However, in the case of multiple dose administration we are expected to give second and subsequent doses before the drug is completely eliminated. Thus ACCUMULATION of the drug should be considered. On repeated drug administration the plasma concentration will be repeated for each dose interval giving a PLATEAU or STEADY STATE with the plasma concentration fluctuating between a minimum and maximum value.
Figure XIV-7, Plot of Cp Versus Time Showing Cp0
We have already looked at the shape of the plasma concentration versus time curve following a single intravenous administration. If we assume instantaneous mixing we start off with an initial concentration Cp0, given by and then we have a fall in concentration with time controlled by the elimination rate constant.
Figure XIV-8, Plot of Cp Versus Time after Two Separate Doses
If the doses are given far enough apart then the concentration will have fallen to approximately zero before the next dose. There will then be no accumulation of drug in the body.
Copyright 2001 David W.A. Bourne