Since many patients with a wide variety of diseases are nowadays stimulated to adopt a physically active lifestyle, the question of the influence of exercise on the pharmacokinetics of drugs has become more and more relevant. Because exercise influences a large number of physiological factors that also determine the pharmacokinetics of drugs, including haemodynamics, metabolism, pH, temperature and gastrointestinal function, it can be expected to have an effect on the pharmacokinetic parameters (absorption, distribution, elimination) of certain agents. However, only a very limited number of studies has been directed towards this issue, and only a very few drugs have been studied. Nevertheless, it is clear that exercise does influence the pharmacokinetics of certain drugs, although the magnitude and direction of the effects vary. This is not surprising in view of the widely differing physicochemical properties of drugs, the many possible - often opposing - effects of exercise on the parameters affecting drug pharmacokinetics and the different types of exercise performed.The chance of a clinically relevant effect of exercise on the pharmacokinetics of a particular drug is largest in those with a steep dose-response curve, a narrow therapeutic range, a need for continuity of therapeutic effectiveness and a relatively short half-life, in combination with intensive exercise of long duration.If untoward drug effects occur during or after exercise, a change in the pharmacokinetics of the drug related to the exercise should be seriously considered as a possible cause.