Special problems and possible errors in drug monitoring were discussed by analyzing three cases of hospital treated patients. The first one showed how easily side effects of drug can be falsely diagnosed and treated as a drug independent complicating disease. Furthermore, the active principle of the drug had to be differentiated from several potentially inducing substances. In the second case, the wrong diagnosis of a drug induced illness caused an intermission of a vital basic therapy. The lateron developing signs were interpreted as symptoms of the primary disease but they might also have been caused by drugs given later. For the third case it should be pointed out that side effects can be provoked during long- or short-term treatment by changing the diet. For drug monitoring it is therefore equally necessary to watch the drugs as well as the living circumstances.