[Evaluation of pharmacokinetic drug-drug-interactions. Critical considerations of the relevance of pharmacokinetic drug-drug interactions of proton pump inhibitors in self medication].

Mechanisms and evaluation of pharmacokinetic drug interactions are discussed in general, including mechanisms beyond the hepatic phase-I reactions, and especially for the example of proton pump inhibitors (PPI), preferentially omeprazole. Particular attention is paid to the use of PPI as self-prescribed drugs. The sequelae of pharmacokinetic drug interactions can be serious. However, only the evidence of clinical consequences will convert such an interaction from a laboratory finding into a possible adverse effect. Without this, interacting drugs can still be co-administered if the specific characteristics of the concerned drugs, quantitative aspects of the interaction, and especially severity and frequency of possible clinical correlates are taken into consideration. It is encouraging that the laboratory findings reported for the PPI--in vitro or ex vivo from volunteer studies--have hardly found equivalents in clinical consequences. As of today, this is also true of the widely discussed interaction with clopidogrel. Regarding the safety of use of PPI as self-prescribed drugs, it also needs to be emphasized that a sizable number of interactions reported for omeprazole and/or pantoprazole were observed at higher dose levels than the 20 mg licensed for self medication. In conjunction with the temporal limitation of PPI self-prescription (14 days), it can be expected that pharmacokinetic drug interactions will generally be no critical factor in the usage of PPI in self-medication. However clinically relevant interactions can occur, e.g. when PPI are combined with extracts from St. John's wort, methotrexat or some inhibitors of HIV-protease with pH-dependent absorption.