Serum Concentrations of Antidepressants in The Elderly

Background: The elderly is the most rapidly growing subpopulation in many countries, and the use of antidepressant drugs among elderly patients may be increasing as depression is one of the most common comorbid conditions in age-related diseases. The aim of the present study was to compare serum concentrations of antidepressants in older versus younger individuals in a naturalistic setting. Methods: Serum concentrations from patients treated with antidepressant drugs were withdrawn from a routine therapeutic drug monitoring database and stratified into the age groups younger than 40 years, 40-65 years, and older than 65 years. Dose-adjusted serum concentrations (concentration to dose ratios; nanomoles per liter per milligram per day) and absolute serum concentrations were compared between the subgroups using patients younger than 40 years as control. Results: Altogether, 32,126 serum concentration samples from 17,930 patients treated with selective serotonin reuptake inhibitors, tricyclic antidepressants, serotonin and noradrenaline reuptake inhibitors, or tetracyclic antidepressant drugs were included. Only minor differences in mean concentration to dose ratios and absolute serum concentrations were observed between patients aged 40-65 years and controls. In contrast, for 12 of the 14 drugs included, approximately 1.5- to 2-fold higher mean concentration to dose ratios were observed in patients older than 65 years versus controls (P < 0.01). Significantly higher absolute serum concentrations were also observed in the former compared with the latter subgroup for 8 of these 12 agents (P < 0.01). Conclusions: Patients older than 65 years had a 1.5- to 2-fold higher exposure of most antidepressant drugs compared with those younger than 40 years when given equal doses. This may indicate an increased risk of concentration-dependent side effects in the elderly.