In programs where therapeutic drug monitoring is accompanied by interpretation of the data, unusual and unanticipated values may mean more than just a laboratory error. Many times such spurious results will be due to unusual circumstances that result in the drug never reaching the bloodstream. In 3 case reports, we show that purposeful inhospital noncompliance and failure to recognize simple drug absorption interactions can be added to the evergrowing list of factors influencing serum concentrations of therapeutic agents.