The familial aggregation and co-aggregation of drug use disorder and alcohol use disorder in siblings of affected individuals born 1950-1990: A birth cohort exposed to rising rates of drug use disorder

Background We seek to clarify how changes in the prevalence of drug use disorder (DUD) in Sweden in the 1950-1990 birth cohort impact the aggregation and co-aggregation in siblings of DUD and alcohol use disorder (AUD). Methods We examined risk for DUD and AUD in siblings of 102,624 DUD cases and matched control probands and 123,837 AUD case and matched control probands identified using Swedish registries. Flexible parametric survival models assessed the difference in disorder risk in siblings of case versus control probands. Results Over birthyears 1950-1990, rates of DUD increased substantially in the Swedish population. In siblings of DUD cases versus controls, the risk for DUD increased dramatically starting in birthyear 1965 while their risk for AUD fell moderately. A similar, but less pronounced pattern, was seen in the siblings of AUD versus control probands. These differences were much larger in male than in female siblings. Conclusions The factors that drove upward population rates of DUD in Sweden (e.g. increased availability, reduced stigma) produced much stronger effects in high-risk subjects (siblings of DUD and AUD probands) than in normal risk groups (siblings of controls), thereby increasing familial aggregation of DUD. However, parallel declines in AUD rates in high-risk versus normal-risk siblings were observed, likely due to 'competitive effects' reducing coaggregation of DUD and AUD. Results of genetic studies of substance use disorders can be substantially impacted by changes in availability and stigma of psychoactive substance use and indirectly by 'competition' as predicted by behavioral economic models, between abusable substances.