Majority of the drug substances are required in multiple doses. In multiple dosing, a drug is administered repetitively with a constant dosing interval such that a steady state of drug concentration in the blood is achieved and maintained.
As shown in the below figure, when the drug concentration declines, another dose is administered to maintain the blood concentration in the therapeutic window. Again when the drug declines after the second dose, a third dose is given to bring up the level of drug and maintain it in therapeutic range. After about the fifth or sixth dose the Cmax and Cmin become constant for each successive dose. This pattern of uniform fluctuations is called the steady state concentration.
At steady state, the rate of drug absorption is equal to the rate of drug elimination. In a multiple dosing regimen involving oral medication, the following equations are useful to find the steady state parameters:
Loading Dose Calculation Formula
Since the steady state conditions are achieved after five or six doses because of accumulation of initial doses, it is likely that the first of those doses have a Cmax which is below the MEC. The second dose may have a Cmax just equal to or slightly above the MEC. In such situations, usually a loading dose is provided which is generally twice the regular or maintenance dose. The loading dose (DL) is calculated as,
Maintenance Dose Calculation Formula
The maintenance dose may be defined as the size of the dose is required to maintain therapeutic range according to the dosage regimen. Maintenance dose (DM) is calculated as,
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