Variability in drug response can be due to either pharmacokinetic or pharmacodynamic factors. The reasons why people differ in pharmacokinetics or pharmacodynamics are manifold and include, e.g., genetic factors, diseases, age and concomitantly administered drugs. Oxidation reactions are dominant in the metabolism of drugs and cytochrome P-450 enzymes (CYP) have been recognized as chief contributors (Guengerich 1992). We have previously shown that drug interactions mediated by the inhibition of CYP enzymes may be of major clinical significance (Olkkola et al. 1993; Olkkola et al. 1994; Varhe et al. 1994; Olkkola et al. 1999; Palkama et al. 1999; Jokinen et al. 2000).