Limitations included the inability to examine mental disorders
that were undiagnosed, thus mental disorders in this study likely
underestimated the true prevalence in the population. This has
an uncertain effect on risk estimates but may cause attenuation
towards the null hypothesis by making the two groups (those with
and without mental disorders) spuriously more similar with
respect to the outcome. Regardless, because we used nationwide
out-patient and in-patient data in Sweden, which has universal
healthcare access, ascertainment was much more complete than
in previous studies. Substance use disorders also were ascertained
using nationwide out-patient and in-patient diagnoses, which
likely underestimated their true prevalence and influence as
mediators. Some suicides may have been misclassified as
accidental deaths,30,31 despite the exclusion of ‘deaths ofundetermined intent’ from the study outcomes. Suicide is a
stigmatising and distressing verdict for families and there can be
pressure to attribute deaths to accidents rather than suicides.
However, the increasing accidental death risk by increasing age
was in contrast to the absence of an age-effect for suicide risk
that we previously noted in this population,23 suggesting that
misclassification was unlikely to be a major influence. Information
on non-fatal accidents was unavailable and would be useful to
examine in future studies when feasible. Finally, some analyses
of specific types of accidents had limited precision because of
small numbers of deaths. Additional large cohort studies will be
needed to further elucidate the risks and mechanisms for different
types of accidents.