This long-term follow-up study examined the association between co-occurring depression and AUD in adolescence and subsequent mental health outcomes in adulthood. To explore this relationship, we analysed longitudinal data from a diagnostically well-characterised and community-representative cohort of young individuals prospectively followed over a 15-year period.

Our findings suggest that individuals with co-occurring adolescent depression and AUD (i.e., dual diagnosis) had a particularly high likelihood of experiencing a negative mental health outcome, which was true for all outcomes examined. Notably, there was a statistically significant difference observed between the dual diagnosis group and those with adolescent depression only in terms of suicidality and AUD. In particular, the risk of continued co-occurring depression and AUD in adulthood was elevated. Individuals who experienced both adolescent depression and AUD also had a significantly higher likelihood of experiencing adult depression and anxiety disorder as compared to those with AUD alone. The observed associations between dual diagnosis and adverse outcomes persisted after adjusting for potential confounding factors, suggesting that the adverse impact on long-term outcomes is compounded by this comorbidity.

Few previous long-term follow-up studies of adolescent depression and AUD extend into early midlife. Our findings partially align with a New Zealand study by Boden & Foulds [23], using a similar design and follow-up period. Similar to their findings, we did not observe a significantly higher risk of adult depression in youth with a dual diagnosis compared to those with only depression. However, our results do suggest that youth with both depression and AUD face a significantly worse prognosis when considering a wider range of outcomes, including suicidality and co-occurring AUD.

What could account for the poorer prognosis of young people with co-occurring conditions in terms of mental health and AUD in adulthood? Previous studies have indicated that individuals with a dual diagnosis exhibit lower treatment adherence and poorer treatment outcomes compared to those with either depression or AUD alone [13, 23]. The comorbidity of depression and AUD may present challenges in achieving successful treatment outcomes [33, 34]. Consequently, this can hinder the individual’s ability to fully recover and lead to a more unfavourable long-term prognosis. Adolescents with ongoing AUD may also be underdiagnosed in the healthcare system [16], potentially leading to a lack of treatment for AUD altogether, further exacerbating the situation. Likewise, depression may be concealed by ongoing AUD, resulting in less effective treatment outcomes in addiction care. Suboptimal care may contribute to a prolonged duration of untreated or undertreated mental health issues, consequently increasing the risk of a negative prognosis.

Considering the cumulative impact on the brain of depression and AUD, it is also conceivable that the burden may be aggravated by the longitudinal course of this early-onset psychiatric comorbidity. Research has shown that AUD during adolescence can have neurocognitive consequences, including behavioural changes, attention deficits, impaired memory, and diminished verbal and visuospatial abilities [35]. Binge drinking, a prevalent pattern of alcohol consumption in Sweden, as well as abstinence and hangover symptoms, have been associated with the most pronounced neurological abnormalities during adolescence, particularly affecting the frontal, parietal, and temporal cortex [36]. Furthermore, a global study using brain imaging techniques has demonstrated widespread cortical changes in individuals with depression, suggesting that depression may dynamically influence brain structure, which may vary across different life stages [20].

Importantly, the social ramifications of depression [18] and AUD [19] are also likely to play a significant role in predicting an unfavourable prognosis. Previously reports based on ULADS suggest that depressed adolescents are at increased risk for a range of adverse social outcomes in adulthood, including labour market marginalisation [37], low income [38], and problems related to intimate relationships [39]. AUD may exacerbate these problems and compound the burden faced by individuals with co-occurring depression and AUD, further impeding their ability to thrive in various aspects of life.

The results of our work suggest dire outcomes for adolescents presenting with co-occurring depression and AUD. This poses the question of how best to clinically manage and treat this vulnerable group, especially within healthcare settings where current literature still reports modest treatment effects [34, 40]. Therefore, future research in this field would benefit from exploring whether timely rigorous treatment, combined (targeting both disorders) or in isolation can change the course of events. Rigorous treatment in this context would refer to combination pharmacotherapy, increased dosages, and possibly more psychotherapy sessions, especially in coordination with social services, where applicable. Furthermore, in view of the heterogeneity and complex pathophysiology of both AUD and depression, emphasis on personalised or patient-centred treatment is warranted [34]. Another area of research that this field could benefit from is an exploration of protective factors for adverse outcomes in this population, including social circumstances that might moderate or mediate the adverse effects of dual diagnosis. Such factors may be capitalised on and strengthened as part of the treatment care plan and possibly for preventive purposes.

A primary strength of this study lies in its long-term perspective, as participants were followed up after they transitioned into adulthood. The comparatively large community sample of adolescents increases the generalisability of the findings. In addition, all participants were assessed using structured diagnostic interviews and validated self-report questionnaires at baseline and at the 15-year follow-up, which undoubtedly is important from a clinical perspective.

The results should be interpreted in light of some limitations. Firstly, ULADS was not originally designed for the research questions addressed here. For instance, the group of adolescents with AUD but no depression during adolescence consisted of few individuals, which implies that any results based on this group must be interpreted with caution. Only individuals identified with depression or suicidality during the initial screening were invited for interviews. Those with only AUD were never called for an interview unless they were included in the control group. We still decided to include this subgroup in the analyses for transparency and to enable future synthesis with similar samples. Overall, the exploratory nature of this study underscores the necessity of independent replication of the findings. Secondly, approximately one-third of the eligible participants dropped out between the baseline and follow-up assessments. However, it is important to note that the dropout rate was similar between the depression group and the control group and there were no statistically significant differences in key variables between those who participated in the follow-up and those who did not. The high proportion of females in the sample reflects the higher prevalence of adolescent depression among females and was not related to attrition. Still, the small number of males prevents us from drawing firm conclusions related to sex-specific patterns. Thirdly, there were no measurement points between the ages of 16 and 30. As a result, some data obtained during the follow-up interview relied on retrospective recall, which may introduce a risk of recall bias [41]. In addition, some outcome measures focused only on the current state at follow-up. Fourthly, although associations were adjusted for a range of child and adolescent characteristics, residual confounding cannot be ruled out. Likewise, we did not investigate the mediating role of psychosocial circumstances in young adulthood. Finally, findings should be interpreted bearing the historical context of this cohort in mind. Global societal trends (e.g., environmental, social, economic, political, or technological change) in combination with declining mental health among youth over the last two decades make the generalisability of our findings uncertain [42]. In addition, alcohol consumption among adolescents has decreased in past decades [43]. Still, we believe the overall pattern of results can be generalised across generations.