Elderly patients suffering from mental diseases are frequently co-medicated with several psychopharmacological agents simultaneously, such as antidepressants, antipsychotics, anxiolytics, hypnotics, and anticonvulsants, used as mood stabilizers. In addition, somatic drugs are co-prescribed for the treatment of other concomitant diseases. This situation increases the risk for pharmacokinetic interactions with pharmacodynamic consequences in a population which is particularly sensitive to adverse effects. Indeed, drug absorption, distribution, metabolism and elimination (ADME) may be altered in the elderly. However, studies in very old patients (_/80 years) are often lacking [1]. Therefore, treatment needs to be carefully and individually tailored. Therapeutic drug monitoring [2] may be a useful tool to optimise treatment, as some ''classical'' indications apply for this population: lack of compliance, adverse effects despite the use of generally recommended doses, suspected drug interactions, combination treatment with a drug known for its interaction potential, patients with pharmacokinetically relevant comorbidities (hepatic or renal insufficiency, cardiovascular disease). The increasing knowledge on the role of cytochrome P-450 isozymes in the metabolism of drugs and their interaction potential has fortunately led to a situation, which allows, to some extent, predicting risks for adverse effects after introducing a polymedication [3_6].