AIM: To investigate whether the effect of alcohol use disorder (AUD) on death by natural and unnatural causes, respectively, differs according to intelligence quotient (IQ) scores.

METHODS: We followed 654 955 Danish men, including 75 267 brothers, born between 1939 and 1959 from their 25th birthday, 1 January 1970, or date of conscription (whichever came last) until 31 December 2018. The exposure of AUD was defined by first registered treatment (diagnosis since 1969, prescription medicine since 1994, or other treatment since 2006), and the outcomes of death by natural and unnatural causes, respectively, were obtained from nationwide registers since 1970. Information on IQ score was retrieved at conscription from the Danish Conscription Database.

RESULTS AND CONCLUSION: In total, 86 106 men were defined with an AUD. AUD combined with the highest, middle, and lowest IQ score tertiles, respectively, were associated with a 5.90 (95% confidence interval [CI] 5.75; 6.01), 6.88 (95% CI: 6.73; 7.04), and 7.53 (95% CI: 7.38; 7.68) times higher hazard of death by natural causes compared with no AUD and the highest IQ score tertile. The risk of death by unnatural causes was comparable for men with AUD regardless of IQ score tertile. A within-brother analysis showed that the impact of AUD on death by natural and unnatural causes, respectively, did not vary between men with different IQ score tertiles, but were hampered by statistical uncertainty. Our study indicates a need of special focus on men with lower levels of IQ score and AUD for prevention of death by natural causes.