The dose-effect relationship can be determined by linking pharmacokinetic drug behavior with information on pharmacodynamic activity. This requires that the quantifiable concentrations of drug (or metabolites) in the plasma (systemic circulation) are related to the concentration at the site of action. Using the pharmacokinetic parameters and a pharmacokinetic-pharmacodynamic model, it is possible to predict the pharmacodynamic response to certain drugs; this provides useful information for understanding drug action and to determine dosage regimen.