The results of clinical laboratory determinations depend on physiological, environmental and pharmacological factors. Thousand pharmacological factors used daily, are known to interfere with routine laboratory tests by pharmaco-toxicological and physico-chemical reactions. This confusing interference may cause the wrong diagnoses but also incorrect treatment, unnecessary tests and further costs. Even if we would be able to recognize the most important effects of drugs, we will not be able to remember all of the drugs adverse reactions; therefore use of computer databases could be helpful for interpreting doubtful laboratory test results.