Unexpected results of laboratory tests may be due to various factors. These include drug effects, which may be either biological, or due to analytical interferences. The more common biological effects include direct and indirect effect produced by drugs in vivo; in consequence laboratory tests yield unexpected, yet correct results. By contrast, the results of tests influenced by the interference of drugs with analytical tehniques are not accurate. International guidelines for the evaluation of drug effects have been formulated, allowing for the comparability of data of different studies. The development of drug effects databases has facilitated the access to the rapidly increasing body of data, which need to be evaluated for their clinical and laboratory significance.