PHARMACOLOGY

MULTIPLE DOSE BIOAVAILABILITY ::

If the drug is administered in repeated does then there may be accumulation of drug if the rate of elimination is slower than the rate of absorption This will continue till the amount eliminated per dose equals the amount administered per dose The characteristic curve will show fluctuations between doses and a plataeu state 

        The concentration at the time of plateau time of achieve plateau and the extent of fluctuation will depend on the half -life of drug Half -life of the drug is the time during which the concentration of the drug is reduced to half (50%) of the plataeu concentration 

The time for plateau depends on half -life and is independent of the dose given it is roughly equal to four half -lives The concentration at plateau is proportional to dose and dosage interval and half-life 

            The fluctuations can be reduced if dose intervals are decreased This is essential in drugs having low therapeutic index e.g. digoxin 

    Although the estimation of drug concentrations in plasma at different time intervals gives a precise picture of bioavailability it is however impracticable to perform such tests for all drugs Attempts have been made to predict bioavailability of different formulations of the same drug through the use of physicochemical parameter as tablet disintegration time and tablet dissolution rate