Pharmacology describes the interaction between drugs and organisms. In this interaction two features are especially important. Pharmacodynamics deals with the effects of a drug on the body; how a drug acts and its side effects, in which tissues, at which receptor sites, at which concentration, etc. The effects of drugs may be altered by other drugs or disease states. Antagonism, synergism, addition and other phenomena are also described by pharmacodynamics. Pharmacokinetics deals with the effects of the body on the drug, through Absorption, Distribution, Metabolism and Excretion (ADME).

The dynamics and kinetics of a drug determine its therapeutic usefulness. The pharmacodynamics of a drug determine its effectiveness and which side effects may occur, and at what concentration. The prescriber has very little influence on this. The pharmacokinetics of a drug determine how often, in what quantity and dosage form and for how long the drug should be given to reach and maintain the required plasma concentration. As the prescriber can actively influence the process, the following section concentrates on this aspect.