Biopharmaceutics is the study of the chemical and physical properties of drugs and the biological effects they produce. Drugs are substances intended for use in the diagnosis, cure, mitigation, treatment, or prevention of disease. Clinical pharmacokinetics is the discipline that describes the absorption, distribution, metabolism, and elimination of drugs in patients requiring drug therapy. Clearance is the most important pharmacokinetic parameter because it determines the steady-state concentration for a given dosage rate. The volume of distribution is a proportionality constant that relates the amount of drug in the body to the serum concentration. Pharmacodynamics is the study of the relationship between the concentration of a drug and the response obtained in a patient. Originally, investigators examined the dose–response relationship of drugs in humans but found that the same dose of a drug usually resulted in different concentrations in individuals because of pharmacokinetic differences in clearance and volume of distribution. Toxicokinetics is an essential step in clinical toxicology. The methodology is based on the same parameters which are used in pharmacokinetics. The mechanism of toxicity is an essential item in the interpretation of toxicokinetic data. By the exact knowledge of toxicokinetics, it is possible to determine for each toxicon, the relevant parameters which will be of use in clinical practice. The dose–response relationship, describes the change in effect on an organism caused by differing levels of exposure to an after a certain exposure time.