Drugs are molecules that interact with macromolecular structures in the body to produce effects that are intended to be beneficial, most often through modification of pathophysiological processes. Some drugs may also be designed to kill intruders, such as bacteria and parasites, or endogenous cells that have lost their growth control and behave as cancer cells. Because a pharmacological effect requires the association of a drug molecule with a receptor structure, one may assume that the more active drug is available at the effect site (biophase), the more effect will be produced. This is basically correct, but reality is more complex as will be shown below when discussing various relationships between drug concentrations and drug effects. The term pharmacokinetic–pharmacodynamic (PK–PD) analysis has been coined to include both the evaluation of pharmacokinetics, which denotes the systematic description of drug transfer through the body, and pharmacodynamics, which means the study and control of drug effects.

Biopharmaceuticals deserve some attention here. At the moment a considerable part of the drugs newly approved by regulatory agencies belong to the so called biologicals. These medicines have a number of characteristics that set them aside from low molecular weight drugs. Their activity can strongly be influenced by their complicated shape based on secondary, tertiary and (sometimes) quaternary structures. These structures cannot be fully defined with our present set of analytical techniques and approaches. They often are the same as (or closely resemble) endogenous proteins. Those are challenging issues but those challenges need to be met and PK/PD studies with biologicals have been published.