Strengths of this study include the use of a very large population-based cohort of patients with OCD and a four-decade study period long enough to capture the outcomes of interest. There were, however, some limitations. First, because suicide is a sensitive issue, it is likely that it is underreported. Suicide may be misclassified as an accident or another cause of death, especially in those who have not been previously diagnosed with a mental disorder. However, in line with previous studies,25, 39 we tried to minimize this issue by including also the events diagnosed as undetermined causes. Second, whereas ICD-10 codes for OCD have excellent validity and reliability, ICD-8 and ICD-9 codes are somewhat less reliable.24 However, only 13% of our patient cohort were diagnosed using these older codes, and stratified analysis showed that the results were largely comparable across the different ICD editions. Third, because outpatient data were only introduced in the Swedish registers in 2001, during years 1969–2000 the cohort only included individuals whose attempted suicide led to inpatient care, potentially leaving out attempts of lower lethality. In addition,
