Many factors contribute to the increased risk for adverse effects in psychogeriatric patients treated with antidepressants and/or antianxiety agents: pharmacoepidemiological studies show that these patients are frequently comedicated with other psychotropic drugs such as antipsychotics, anticonvulsants used as mood stabilizers, but also with somatic drugs. The presence of both somatic and psychiatric comorbidities results in the implication of several prescribers. Unfortunately, coordination between the prescribersmay not be optimal and this contributes to increase the risk for adverse effects in a population, which is particularly sensitive to adverse effects (Green, Am J Geriatr Pharmacother 5 (2007) 31). Indeed, drug absorption, distribution, metabolism and elimination(ADME)may be altered in the elderly (Schwartz, Clin Pharmacol Ther 82 (2007) 87) and pharmacokinetic interactions may have particularly severe consequences (Spina and Scordo, Drugs Aging 19 (2002) 299). Most psychotropic drugs are substrates and/or inhibitors of cytochrome P-450. The increased knowledge about this enzymatic system allows a better understanding of underlying mechanisms responsible for pharmacokinetic interactions responsible for adverse effects. In this context, therapeutic drug monitoring represents a useful tool to optimise treatment, as many generally admitted indications apply for elderly patients (Baumann et al., Pharmacopsychiatry 37 (2004) 243): 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). On the other hand, there is a lack of experimental evidence as clinical pharmacological studies in elderly patients are relatively rare.