Antipsychotic load during olanzapine mono- versus polytherapy and in relation to age

While antipsychotic monotherapy is generally recommended in treatment of psychotic disorders, polytherapy is frequently observed in clinical practice. We aimed to investigate the total antipsychotic load among patients treated with olanzapine in monotherapy vs. polytherapy, and how the antipsychotic load changed in relation to age. Patients with serum concentration measurements of olanzapine alone (monotherapy) or in combination with other antipsychotics (polytherapy) were retrospectively included from a therapeutic drug monitoring service. The antipsychotic load was measured as olanzapine dose and concentration equivalents. Patients were categorised into three age groups (18–49 years, 50–74 years and ≥ 75 years of age). In total, 10 190 patients (54% males) were included, whereof 8 759 (86%) received olanzapine in monotherapy and 1 431 (14%) in polytherapy. The mean olanzapine dose equivalent was significantly higher in the polytherapy group compared to the monotherapy group (23.0 vs. 11.1 mg, p < 0.001). Likewise, the mean concentration equivalent was higher in the polytherapy group than in the monotherapy group (179 vs. 89.1 nmol/L, p < 0.001). When comparing patients ≥ 75 years with patients 18–49 years, the mean dose equivalents were 43% and 41% lower in the monotherapy and polytherapy group, respectively (both p < 0.001), whereas the respective concentration equivalents were only 11% (p < 0.001) and 10% (p = 0.141) lower. Patients receiving olanzapine in antipsychotic polytherapy had twice as high antipsychotic load compared to patients using olanzapine monotherapy, both in terms of olanzapine dose and concentration equivalents. Although the antipsychotic doses were reduced by age, the concentration levels remained similar.