Summary The main feature of observational studies is the representation of naturalistic treatment conditions. In contrast to clinical trials, they allow the evaluation and quantification of adverse event profiles of drugs under “real life” conditions. The price for this unquestionable chance is the proneness to distorting factors, which may aggravate the interpretation of the study results. Analysis of observational study results therefore has to control for potentially influential factors and reconsider possible alternatives explaining observed associations. The most important distorting factors, which should be taken into account during analysis and interpretation are under-reporting, event selection, bias, confounding and misusage? Authors and readers of such study results should be aware of this possible sources of error, in order to derive optimal benefit from this study approach.