Forensic toxicology deals with the evaluation of plasma concentrations of foreign substances in the human body. Administered doses are estimated and blood concentrations at the time of the offence are evaluated, for example. Genetically caused changes of metabolizing enzymes (pharmacogenetics) as well as induction or inhibition of enzymes involved (pharmacokinetics) can affect plasma concentrations and should be taken into account in the assessment. While specialist literature can be consulted in the daily routine, expert witnesses are expected to answer corresponding questions immediately when questioned in court. This paper presents an overview of important pharmaceutical drugs and illegal drugs including their degradation pathways and well-known polymorphisms. Beyond that, the possibility of potential interactions is assessed and further presented in a chart for practical use in foro.