Genetic and environmental factors contribute to a wide inter- and intraindividual variability in drug metabolism. Among the environmental factors that may influence drug metabolism, the diet and nutritional status of the individuals are important determinants. As altered drug-metabolising enzyme activities can influence the intensity and duration of drug action, such factors should be considered in pharmacotherapy. For this reason the effects of dietary energy, protein deficiency, nutritional ingredients, special diet forms and nutrition regimens and malnutritional states must be differentiated. In various pharmacokinetic studies different model drugs metabolised either by oxidative phase I pathways [e.g. phenazone (antipyrine), aminopyrine, phenacetin, theophylline, propranolol, nifedipine] or phase II conjugation reactions [e.g. paracetamol (acetaminophen), oxazepam] were used and from the calculated pharmacokinetic data some information on the involved and affected drug-metabolising enzymes [e.g. cytochrome P450 (CYP) subspecies, glucuronosyltransferases] can be generated. It is well known that smoking, charcoal broiled food or cruciferous vegetables induce the metabolism of many xenobiotics, whereas grapefruit juice increases the oral bioavailability of the high clearance drugs nifedipine, nitrendipine or felodipine by inhibiting their presystemic (intestinal) elimination. Energy deficiency, and especially a low intake of protein, will cause a decrease of about 20 to 40% in phenazone and theophylline clearance and elimination of those drugs can be accelerated by a protein-rich diet. In the same way, protein deficiency induced by either vegetarian food or undernourishment will have the opposite pharmacokinetic consequences. On the basis of some more examples from the literature it is emphasised that the variable influence of the above factors should be taken into account in study participant selection and study design when the pharmacokinetics of a drug must be determined in healthy individuals and/or patients.