The main exposure variable was defined as first contact due to OCD. Using the Danish Psychiatric Central Register9 and the Danish National Patient Register,10 we identified all patients who received a diagnosis of OCD based on the ICD-8 (code 300.39) and the ICD-10 (code F42). These registers comprehensively represent psychiatric hospital admissions to inpatient and outpatient facilities because private psychiatric hospitals do not exist in Denmark. The date of onset was defined as the day of the first contact for the diagnosis in question.
