ABSTRACT
Our considerations thus far have made it obvious that many factors play a role in determining the biologic effects observed when a chemical agent interacts with a living organism. The more complex the biologic system, the greater the number of variables that may affect the ultimate response. Some factors influence the nature, or pharmacodynamics, of the drug response. Sometimes these result in effects such as allergic responses that are qualitatively different from those for which the drug is usually administered. Most factors that modify the effects of drugs, however, do so by altering their pharmacokinetics and, thereby, the intensity of the response. Once recognized, these quantitative changes in the usual effects of drugs can be corrected by appropriate dosage adjustment. We have already discussed the most important factor in determining the intensity of a biologic effect in any individual-the dose of drug administered. But the intensity of drug effect is also dependent on how much of a dose reaches a site of action at any one time and how long the drug remains there in effective concentrations. Thus it follows that even for a single dose of a single agent, the variability in the intensity of response among different individuals may result from the influence of the many factors that affect absorption, distribution, biotransformation and excretion.
