Association between body mass index and serum concentrations of olanzapine and selective serotonin reuptake inhibitors in patients with eating disorders.

PURPOSE: Selective serotonin reuptake inhibitors (SSRIs) and olanzapine are frequently prescribed for patients with anorexia nervosa and bulimia nervosa. The effect of severely low body mass index (BMI) on the pharmacokinetics of these drugs has received little attention. The aim was to investigate the association between BMI and dose-adjusted serum concentrations (concentration/dose ratios; CDRs) of SSRIs and olanzapine in patients with eating disorders. METHOD: We conducted a retrospective analysis of patients from a regional center for eating disorders. BMI, daily doses of olanzapine and SSRIs, and their serum concentrations were obtained from medical records. Linear regression analyses were conducted with CDR for each drug as dependent variable and BMI as independent variable. A group of patients with schizophrenia or bipolar disorder was included for comparison of patients with BMI in the normal range. RESULTS: Sixty-five patients with eating disorders and 545 patients from the comparison group were included. Among the patients with eating disorders, there were no significant associations between BMI and CDRs for any of the drugs. CDR for olanzapine was significantly higher in the eating disorder group than in the comparison group, but most of this difference could be attributed to the higher proportion of smokers in the comparison group. CONCLUSION: This study suggests that patients with eating disorders do not have clinically important changes in CDRs of SSRIs and olanzapine related to their low BMI. Although reassuring, the number of included patients treated with each drug is low.