ABSTRACT
It bears repeating that one of the most important principles of pharmacology is that the magnitude of drug effect is a function of the concentration of drug at the site where the action-effect sequence is initiated. As we have seen, the dose is a critical factor in determining the intensity of the response. The proportion of the dose actually contributing to the concentration at an active site is the net result of many processes occurring simultaneously: (1) the rate and extent of absorption from the site of application; (2) the rate and extent of tissue distribution; (3) the rate of biotransformation to active or inactive metabolites; and (4) the rate of excretion (Fig. 10-1). If we set aside other differences that exist among individuals, it follows that the variability in the intensity of the biologic effects elicited in response to a single dose of an agent results from differences in absorption, distribution and elimination (biotransformation and excretion) among individuals.
