An association between autumn birth and clozapine treatment in patients with schizophrenia: a population-based analysis.
BACKGROUND
Numerous studies on seasonality of birth and schizophrenia risk have been published but it is uncertain whether, among those with schizophrenia, refractory illness exhibits any predilection for birth month. We hypothesized and examined whether a season of birth effect was present in patients with schizophrenia with a history of clozapine treatment.


METHODS
Using record linkage with Danish registers, we examined patients with schizophrenia born between 1950 and 1970, and between 1995 and 2009 and Cox regression analysis was used to examine season of birth in relation to history of clozapine treatment.


RESULTS
In a study population corresponding to 60,062 person-years from 5328 individuals with schizophrenia of which 1223 (23%) received at least one clozapine prescription, birth in the autumn (September-November) was associated with clozapine treatment (HR = 1.24; 95% CI 1.07-1.46) when compared with birth in the spring (March-May).


CONCLUSIONS
Although replication studies are needed, this is the first evidence from a nationwide study suggesting a possible season-associated risk of clozapine treatment in schizophrenia. The reasons for this relationship remain to be further investigated but might be partially explained by early exposures such as winter flu season and low vitamin D levels.
Holger J. Sørensen, Centre for Schizophrenia, Aalborg University Hospital, and Psychiatric Centre Copenhagen , Capital Region of Copenhagen , Denmark.
Holger J
HJ
Sørensen
Leslie
L
Foldager
Rasmus
R
Røge
Sofie Gry
SG
Pristed
Jesper T
JT
Andreasen
Jimmi
J
Nielsen
D000328:Adult; D014150:Antipsychotic Agents / Q000627:therapeutic use*; D003024:Clozapine / Q000627:therapeutic use*; D004363:Drug Utilization / Q000706:statistics & numerical data; D005260:Female; D006801:Humans; D008297:Male; D008498:Medical Record Linkage; D008875:Middle Aged; D018570:Risk Assessment / Q000379:methods; D012559:Schizophrenia / Q000188:drug therapy*; D012621:Seasons
D014150:Antipsychotic Agents; D003024:Clozapine
Clozapine; Population; Risk factor; Schizophrenia; Seasonal birth pattern