Psychogeriatric patients frequently suffer from comorbidities such as dementia, depression, and somatic diseases. In addition, patients with dementia often display a heterogeneous group of symptoms, the Behavioral and Psychological Symptoms of Dementia (BPSD), for which both pharmacotherapy with several drugs and non-pharmacological treatments are recommended. They are frequently comedicated with different psychopharmacological agents including antidepressants, antipsychotics, anxiolytics, hypnotics, cognitive enhancers, and Li, antipychotics and anticonvulsants used as mood stabilizers. In addition, somatic drugs are co-prescribed for the treatment of other concomitant diseases. As a consequence, these patients, who are characterised by an increased sensitivity to adverse effects, by a lowered homeostatic reserve and by decreased compliance are nonetheless often exposed to complex drug regimes. Such patients are therefore vulnerable to pharmacokinetic and pharmacodynamic interactions with severe clinical consequences. Studies in very old patients (> 80 y) are often lacking. Therefore, treatment needs to be carefully and individually tailored. Therapeutic drug monitoring 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.