Long-term persistence of the risk of agranulocytosis with clozapine compared with other antipsychotics: a nationwide cohort and case–control study in Finland

Background Agranulocytosis is a life-threatening side-effect of clozapine, the only approved drug for treatment-resistant schizophrenia. The long-term profile of this complication has not yet been well established. Here we aim to describe the risk of clozapine-induced agranulocytosis over the long term. Methods We used the entire population of Finland to identify people diagnosed with schizophrenia or schizoaffective disorder between 1972 and 2014 and developed a Kaplan–Meier model of time to diagnosis of agranulocytosis during clozapine versus non-clozapine treatment over a 22-year observation period (1996 to 2017). Next, we developed a nested case–control model for agranulocytosis matching by sex, age, time since diagnosis, and being in the incident cohort on a 1 to 5 ratio. Various durations of use for clozapine and non-clozapine antipsychotic treatment were compared to the modal antipsychotic use duration, deriving adjusted odds ratios (aORs) in a multivariable regression model. Recurrence and lethality rates for clozapine-induced agranulocytosis were described. These data reflect on all individuals with lived experience of schizophrenia in Finland during the study time, although individuals with lived experience were not included in the design of the study. Findings We identified 61 769 people with schizophrenia or schizoaffective disorder (14 037 individuals treated with clozapine and 47 732 individuals treated with non-clozapine antipsychotics), with a mean age of 46·67 years (IQR 34·44–57·61), of whom 30 721 (49·7%) were female and 31 048 (50·3%) were male (data on ethnicity not available). Among those, 398 individuals were diagnosed with agranulocytosis (231 individuals treated with clozapine and 167 individuals treated with non-clozapine antipsychotics), representing a cumulative incidence of agranulocytosis for 1·37% (95% CI 0·58–3·16) on clozapine and 0·13% (0·04–0·23) on non-clozapine antipsychotics. In the case (n=398) versus control (n=1987) model, the risk of clozapine-induced agranulocytosis decreased steeply over time from an aOR of 36·01 (95% CI 16·79–77·22) for less than 6 months on clozapine to 4·38 (1·86–10·34) for clozapine use of 54 months or more. Only one of 3559 individuals starting clozapine died because of clozapine-induced agranulocytosis. Interpretation The risk of clozapine-induced agranulocytosis decreases steeply over time but might be persistently greater than that of non-clozapine antipsychotics. This long-term risk excess seems small in absolute terms compared with the known magnitude of the advantages of clozapine in relevant outcomes, including life expectancy. Given the widespread underuse of clozapine, relaxing the long-term neutrophil monitoring could favour the advantages of long-term clozapine use, including greater life expectancy, without incurring the intolerable risk of clozapine-induced agranulocytosis. Funding Northwell Health and Sigrid Jusèlius Foundation.