Objective: The aim of the study was to identify the pattern of usage of clozapine in Christchurch, New Zealand, including daily dose, indication and use of drug concentration monitoring.

Method: Patients (n=353) were identified retrospectively from the pharmacy computer system. Data gathered included patient demographics, the daily clozapine dose and the number of occasions that clozapine drug concentration monitoring occurred. In addition, each psychiatrist who had prescribed clozapine was surveyed, regarding their indications for the use of clozapine and their use of clozapine drug concentration monitoring.

Results: The majority (63%) of patients on clozapine were male. The mean age of the patients was 43 years (range 15–88 years). The mean daily dose of clozapine was 325 mg (range 12.5–900 mg). Patients over the age of 65 years were on a significantly lower dose (mean=143 mg, 95% CI=103–183 mg) compared with those under 65 years of age (mean=350 mg, 95% CI=330–370 mg). The median duration of treatment on clozapine was 4 years. Fifty-one percent of patients had undergone drug concentration monitoring, the majority on multiple occasions. In females, increasing age correlated with an increase in dose-corrected plasma clozapine concentrations (r²=0.29, p<0.001). This was not demonstrated in the male population. Of the psychiatrists surveyed, 44% prescribed clozapine for unlicensed indications and 79% used clozapine drug concentration monitoring in their patients. This was most commonly performed to assess compliance or confirm toxicity.

Conclusions: The mean daily dose of clozapine of 325 mg was similar to that found in other studies. An age-related decline in dose was observed, probably due to different indications, with many of the elderly patients receiving clozapine for Parkinsonian related symptoms. There was also an age-related decline apparent in clearance in females. Clozapine was often used for unlicensed indications, and a clear majority of psychiatrists use drug concentration monitoring.