BACKGROUND Bipolar disorder (BD) is associated with increased mortality, but evidence on cause-specific mortality is limited.

OBJECTIVE To investigate cause-specific premature excess mortality in BD.

METHODS Finnish nationwide cohort study of individuals with and without a diagnosis of BD who were aged 15-64 years during 2004-2018. Standardised mortality ratios (SMRs) with 95% CIs were calculated for BD using the mortality rates in the Finnish general population without BD as weights. Causes of death were defined by the International Classification of Diseases, 10th revision codes.

FINDINGS Of the included 47 018 individuals with BD, 3300 (7%) died during follow-up. Individuals with BD had sixfold higher mortality due to external causes (SMR: 6.01, 95% CI: 5.68, 6.34) and twofold higher mortality due to somatic causes (SMR: 2.06, 95% CI: 1.97, 2.15). Of the deaths due to external causes, 83% (1061/1273) were excess deaths, whereas 51% (1043/2027) of the deaths due to somatic causes were excess. About twice the number of potential years of life were lost in excess due to external causes than due to somatic causes. Alcohol-related causes contributed more to excess mortality than deaths due to cardiovascular disease.

CONCLUSION External causes of death contributed more to the mortality gap than somatic causes after controlling for age-specific background general population mortality.

CLINICAL IMPLICATION A balanced consideration between therapeutic response, different treatment options and risk of cause-specific mortality is needed to prevent premature mortality in BD and to reduce the mortality gap.