The presentation calls the attention to the hidden threat of drug interactions, which has not been recognized and resolved yet in the Czech Republic. The author names the modes of interactions of particular medicaments, describes the importance and variety of particular cytochromes in metabolism of medicaments and calls the attention to the differences of metabolism in some people. The selected drug interactions are described: simvastatine-diltiazeme, warfarine-propaphenone, warfarine-amiodarone and SSRI-NSAID. Even though the PC programmes which may help physicians to identify interactions are available, the important role may be played by health insurancecompanies whichmay identify the mostcommonserious drug interactions in particular region and in particular physicians.

        Key words: drug interactions – quality of health care – cytochrome P450 – support of effective pharmacotherapy