Both between and within countries, Indigenous’ Peoples represent diverse groups with unique perspectives on health and wellbeing. Broadly, Aboriginal and Torres Strait Islander people in Australia view health with the perspective that a holistic and whole-of-life approach is needed to achieve positive health and wellbeing outcomes (Australian Government, 2017). This sentiment is echoed by Indigenous groups in the United States (US), Canada, and Aotearoa (New Zealand), where physical health is linked with mental, social, and spiritual wellbeing (Gall et al., 2021; Hodge et al., 2009; Mark & Lyons, 2010). Colonisation continues to impact the health of Indigenous people through ongoing intergenerational trauma and discriminatory systemic policies (Australian Institute of Health & Welfare, 2022; O’Neill et al., 2018; Te Kōmihana Whai Hua O Aotearoa (New Zealand Productivity Commission), 2022). This impact is apparent through the disproportionate prevalence of mental health (MH) conditions and severity of substance use among Indigenous people. In Australia in 2011, MH conditions and substance use disorders contributed to 23% of the total burden of disease for Aboriginal and Torres Strait Islander people, the largest burden out of any disease group (Australian Institute of Health & Welfare, 2016). Similar overrepresentation of MH conditions and substance use has been reported for Maori people in Aotearoa (Ministry of Health New Zealand, 2018). Reducing the impact and severity of MH conditions and substance use is important for improving the health and wellbeing of Indigenous people.

While previous research has examined the published literature examining MH conditions or substance use among Indigenous peoples (Kisely et al., 2017; Nelson & Wilson, 2017), the scope of research examining the co-occurrence of these conditions is less clear. Co-occurring conditions often have an exacerbating and debilitating impact on the health of affected individuals (Leung et al., 2016). Co-occurrence is associated with greater severity of MH symptoms, higher quantity of substance use, increased disability or impairment, poorer social functioning, and increased risk of suicide (Burns & Teesson, 2002; Leung et al., 2016; Quello et al., 2005). Co-occurring conditions have also been reported to significantly impact family and carers (Biegel et al., 2007; September & Beytell, 2019). Considering co-occurring conditions, as opposed to individual conditions, provides a more person-focussed and integrated approach to wellbeing (Mercer et al., 2016). Integrated treatment of co-occurring conditions has also demonstrated improved treatment outcomes (Leung et al., 2016). Despite this, most research and treatment provision continues to adopt an approach where researchers and clinicians focus on singular conditions (McCartney, 2016; Teesson et al., 2014).

To the authors’ knowledge, no previous review has systematically examined the literature for co-occurring MH conditions and substance use among Indigenous’ people, nationally or internationally. Scoping reviews allow exploration of the volume and coverage of particular topics to identify gaps, concepts, and key characteristics, as well as informing the feasibility of more detailed systematic reviews (Munn et al., 2018). The number of research outputs in an area of research can be considered a proxy for resource and investment in a particular field (Ebadi & Schiffauerova, 2016), allowing funding bodies to identify areas where research activity is lacking. Furthermore, examining the scope of the literature, such as location, design, setting, and focus, will allow the identification of research gaps to inform future research and policy directions regarding co-occurring MH conditions and substance use among Indigenous peoples (Munn et al., 2018).

Aims

To systematically review studies examining co-occurring MH conditions and substance use among Indigenous peoples globally to determine the scope of publications in terms of (i) volume over time, (ii) MH conditions and substance conditions examined, (iii) countries where the research was undertaken, (iv) research designs implemented, and (v) included Indigenous groups and settings.

Methods

Methodology is reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) scoping review guidelines (Page et al., 2021; Tricco et al., 2018).

Indigenous Leadership for This Review

This review concept arose through collaboration between Indigenous and non-Indigenous researchers and those working in the health field on an Australian Rotary Health Fellowship to understand the co-occurrence of MH conditions and substance use among Aboriginal and Torres Strait Islander Peoples. Co-authors JR, TH, RD, JN, and BR are Aboriginal members of the team and formed the Aboriginal Advisory Group for this project. In addition to lived experiences, the Aboriginal Advisory Group has expertise in mental health, substance use, and health services. The remaining authors are non-Indigenous. The aims of the review were discussed and revised through ongoing collaborative discussion with the whole research team. Study methodology and interpretation of results were discussed and reviewed by Indigenous and non-Indigenous team members throughout the project.

Search Strategy

A medical librarian was consulted to develop and implement the search strategy. Medline, Embase, PsycInfo, and Web of Science were searched across all years up until 11th October 2022. Key terms included “mental disorders”, “substance related disorders”, “comorbidity” and “Indigenous peoples”. A full list of the search terms used can be found in Appendix 1. A manual review of the reference lists of included papers was performed to identify any additional articles not identified through the search strategy.

Inclusion and Exclusion Criteria

Studies were included in the review if they:

  1. (i)

    Focussed on Indigenous peoples. Studies were identified through the use of terms such as “Indigenous”, “First Nations”, “Aboriginal” or referring to the name of a specific Indigenous peoples (e.g. Maori). If the research included both Indigenous and non-Indigenous peoples, comorbidity data needed to be presented separately for the Indigenous cohort or more than 50% of the sample needed to be Indigenous to be included. This criterion was intended to reduce the number of studies that incidentally included Indigenous peoples but did not specifically explore comorbidity among these groups.

  2. (ii)

    Reported on comorbidity across at least one MH condition and one type of substance use. The type of comorbidity examined, and the determination of comorbidities was deliberately broad to enable a more inclusive scoping approach (see Box 1).

  3. (iii)

    Described data-based research, including experimental, quantitative, and qualitative studies.

Box 1 Categorisation of conditions.

• Mental health conditions: Were broadly mapped to the International Classification of Diseases (ICD); however, given condition groupings differ across versions, it was difficult to map papers published before the ICD-11 (World Health Organization, 2019) (current version published in 2019). The broad disorder groupings for this paper included anxiety disorders (e.g. generalised anxiety disorder, panic disorder), depressive disorders, bipolar disorder, stress-related disorders (e.g. post-traumatic stress disorders), schizophrenia/psychotic disorders, eating disorders, personality disorders, and behavioural and emotional disorders (e.g. attention deficit/hyperactivity disorder, conduct disorder). A category was also included for multiple conditions being grouped together or papers that examined general mental health (e.g. only reported “mental health disorder”)

• Alcohol or other drug use: Substances were mapped to the ICD-10 list of substance use disorders, including alcohol, nicotine, cannabis, cocaine, stimulants including amphetamine and methamphetamine, opioids, hallucinogens, inhalants, sedatives, and grouped/multiple substances (World Health Organization, 1993)

Studies were excluded if they:

  1. (i)

    Exclusively reported on suicide, suicide ideation, suicide attempts, and learning disorders as these are not considered MH conditions.

  2. (ii)

    Exclusively focussed on gambling or gaming addictions as these are not substance-based. Studies which included gambling or gaming addiction and substance addiction were included, but only substance data were extracted.

  3. (iii)

    Focussed on basic sciences (e.g. genetic studies).

  4. (iv)

    Were commentaries, case studies, conference abstracts, theses, or reviews.

  5. (v)

    Were published in a language other than English.

Article Screening

Paper titles and abstracts were reviewed against the inclusion criteria by three authors (BH, JR, and JYO) and a research assistant. The first 108 papers were independently coded by two researchers (BH and JR or the research assistant) and checked for agreement. There was an 88% agreement rate between coders after initial coding. Discrepancies were discussed until consensus was reached. Following this, JR and the research assistant screened the remaining studies. All excluded studies were cross-checked by JYO at a later date. BH and JYO reviewed the full texts for final inclusion.

Data Extraction

A data extraction template was created using REDCap electronic data capture tools (Harris et al., 2019). This was piloted by JB, MF, and JYO with BH making amendments before commencing full data extraction. The following data were extracted from the included studies: author, year, conditions examined and how conditions were assessed (e.g. diagnostic interview), country, Indigenous population group, research design, sample size, and setting. Studies were categorised into experimental, qualitative, or quantitative descriptive research designs. Experimental studies were then coded as randomised-controlled trials, non-randomised controlled trials, interrupted time series, or controlled before and after studies (Cochrane Effective Practice & Organisation of Care, 2017). Study setting was categorised based on anticipated settings for MH conditions and substance use research (e.g. general community, Indigenous communities, MH or substance use services, hospitals), with author derived coding for additional settings (e.g. legal settings, such as detention centres). Where multiple papers were derived from one study, data were extracted for each individual paper due to the different combinations of comorbidities examined across papers resulting from large datasets.

Data Analysis

Frequencies and proportions were used to synthesise the extracted data. A linear regression was performed to examine changes in the volume of publications over time, with a p-value of < 0.05 used to indicate significance.

Results

Figure 1 presents the results of the literature search using the PRISMA flow chart. A total of 6049 citations were identified through the search strategy and 4 through searching reference lists, with 4534 articles retained following the removal of duplicates. A total of 3979 articles were excluded at the title and abstract screening stage, and 555 underwent full text review. Ninety-four articles met the inclusion criteria and were included in the review. For brevity, the frequencies of studies are presented in-text without specific references. The Supplementary file (S1), however, provides the detailed data extraction for each included paper.

Fig. 1
figure 1

Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) flow chart

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Volume of Publications Over Time

While publications demonstrated a statistically significant increase over time from 1990 to 2022 (R2 = 0.26, p-value = 0.003), the volume of publications was low across most years. An average of 2.8 papers were published per year since 1990. From 1990 to 2000, there were an average of 1.3 publications per year (n = 14 total), which increased from 2001 to 2011 and 2012 to 2022, with an average of 3.4 (n = 37 total) and 3.8 (n = 42 total) per year, respectively (see Fig. 2).

Fig. 2
figure 2

The number of studies examining comorbid mental health conditions with alcohol and other drug use by year

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Co-Occurring Conditions Examined

Studies included an average of 2.4 MH condition groupings, and 44 papers examined three or more MH condition groupings. Among these, depressive disorders (n = 64, 68% of studies) and anxiety disorders (n = 49, 52%) were the most commonly examined, and eating disorders were the least frequently examined (n = 6, 6%). MH conditions were primarily measured using diagnostic interviews (n = 55, 59%), of which 2 studies used validated screening measures in addition to diagnostic interviews. Of the remaining studies, 20 (23%) used validated screening measures, 5 (5%) used non-validated screening measures, and 11 (12%) used medical records. One (1%) study did not report how MH conditions were measured.

For substance use, studies included an average of 1.9 substances, and 25 papers examined three or more substances. Alcohol was the most frequently examined substance (n = 58, 62% of studies), followed by general substance use or grouping of multiple substances (n = 45, 48%). Sedatives (n = 3, 3%), hallucinogens (n = 3, 3%), and inhalants (n = 2, 2%) were the least frequently examined substances. Substances were primarily measured using diagnostic interviews (n = 44, 47%), of which one also used non-validated screening measures. Of the remaining studies, 20 (22%) used standardised screening measures, 13 (14%) used non-validated screening measures, one (1%) study used both validated and non-validated screening measures, 11 (12%) used medical records, and two (2%) used help-seeking to indicate substance use. Three studies did not report how MH conditions were measured.

There were a broad range of co-occurring conditions examined (see Table 1). It was also common for papers to group MH or substance use conditions or to consider these in general terms (e.g. reporting on “substance dependence” or “MH conditions”). Alcohol and depression were the most frequently examined co-occurring conditions (n = 17).

Table 1 Number and citations for combinations of co-occurring conditions within included studiesa
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Countries Where the Research Was Undertaken

Most studies were conducted with Indigenous peoples from the US (n = 67, 71%). Of these, 46 included Native American Indian Peoples, two studies included Indigenous peoples from Alaska, and 12 included both Native American Indian and Alaskan participants. Four studies included Indigenous peoples from the US and Canada, and three studies specifically included Native Hawaiian and other Pacific Islander Peoples. One study included Native American Indian, Alaskan, and Hawaiian people. Fewer studies were conducted in Australia with Aboriginal and Torres Strait Islander people (n = 13, 14%); Canada among Indigenous Metis and Inuit people (n = 8, 9%); and Aotearoa of which five included Māori people and one included Pacific Islander people (n = 6, 6%). While the Pacific Islander groups (i.e. Samoan, Tongan, Cook Islanders) in the latter study are not Indigenous to Aotearoa, they are Indigenous to the Pacific Islands, and therefore, this study was retained. Two studies included Han Chinese people, one study included Malayo-Polynesian Aboriginal people from Taiwan, and one study included Malay people from Malaysia (see Fig. 3).

Fig. 3
figure 3

The number of studies examining comorbid mental health and alcohol or other drug use by country. Note: Four studies included Indigenous people from the United States and Canada and have been counted for both countries

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Research Designs Implemented

Almost all publications used quantitative descriptive designs (n = 91, 97%). One study used a mixed method design, including a qualitative phase followed by a randomised controlled trial. One study used a randomised controlled trial design, and one study used a qualitative design. Among the descriptive studies, 79 (87%) used a cross-sectional design, nine (10%) used a longitudinal design, and two (2%) used a cross-sequential design.

Indigenous Sample and Settings

Sixty-nine (73%) of the included studies included only Indigenous participants, while the remaining 25 (27%) included both Indigenous and non-Indigenous people. Only one study had a sample where more than 50% were Indigenous with the outcomes combining both Indigenous and non-Indigenous data (Woodall et al., 2007). The remaining studies that included non-Indigenous participants reported the findings separately for the Indigenous participants. The sample ranged from 15 to 16,640 Indigenous participants, with an average of 1387. Most studies were conducted within Indigenous communities/reservations (n = 33, 35%) or a general/population-based sample (n = 28, 30%), of which two studies were conducted within the general population and a reservation (see Fig. 4).

Fig. 4
figure 4

The number of studies examining mental health conditions with alcohol and other drug use by setting. Note: Some studies included multiple settings and have been counted multiple times. *“Other” settings included a homeless community, veteran affairs services, education facilities, and a remote area mental health service database

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Discussion

The number of studies examining co-occurring MH conditions and substance use in Indigenous peoples has increased over time; however, the average number of publications per year was low. This modest increase does not reflect the growth in research outputs in Indigenous health generally in recent decades (Bryant et al., 2022; Derrick et al., 2012; Kennedy et al., 2022), which has been coupled with growing investment and capacity in the MH field for Indigenous peoples in several countries. A recent review of Aboriginal and Torres Strait Islander health in Australia found that 20.5% of all Indigenous health research conducted from 2008 to 2020 was focussed on MH or substance use disorders (Kennedy et al., 2022). There was a peak for paper outputs in 2006. These publications reported on studies that were conducted across a range of years, from 1994 to 2004. Furthermore, nine papers published in 2006 reported the outcomes for four individual studies. Although it is difficult to interpret the reasons for the 2006 peak in paper publications, it may reflect researchers’ capacity to publish in a timely manner rather than any policy or funding factors.

Included studies explored a range of comorbidities. It was common for studies to include generalised groupings of MH conditions or substance use. The co-occurrence of depression and alcohol use was the most examined comorbidity, which likely reflects the high prevalence of both of these conditions (GBD 2016 Alcohol Collaborators, 2018; Liu et al., 2020). There has been limited exploration of substance use with MH conditions such as eating disorders or bipolar disorders. Similarly, MH comorbidities with opioids and stimulants have been limited, although it is likely that these substances have been included in some studies which have explored general substance use. There is significant scope for research exploring several co-occurring MH conditions and substances among Indigenous people, for both prevalence and prevention or treatment strategies. The method of measuring conditions is an important consideration for future research to ensure accuracy in the data provided. While measurement of MH conditions and substance use varied across the studies, this was primarily conducted via diagnostic clinical interviews. This method provides clinical accuracy; however, it may not account for subthreshold symptoms, which have been found to be a significant contributor to burden and disability (Rai et al., 2010). The use of validated MH and substance use measures, which allow for exploration of different levels of severity, may therefore be more relevant for understanding comorbidity burden in Indigenous communities rather than via diagnostic interviews.

A substantial majority of the studies were conducted within the US. The output across countries can likely be attributed to variation in research capacity and support. The US was ranked number one for research outputs from 1996 to 2021, far surpassing Canada, Australia, and Aotearoa’s outputs combined (SCImago, 2022). In addition, leading US mental health groups have advocated for addressing co-occurring MH conditions and substance use, as well as a need for integrated health systems (Mental Health America, 2017; Substance Abuse & Mental Health Services Administration, 2022). This review highlights the need for increased research capacity in co-occurring MH conditions and substance use within distinct and unique Indigenous communities in most countries.

Experimental research is important for providing robust evidence for the effectiveness of treatment approaches. However, only two experimental studies (2%) were identified in this review. This finding is similar to a recent review that found only 2.7% of research on Aboriginal and Torres Strait Islander health in Australia used an experimental design (Kennedy et al., 2022). Qualitative research was also only utilised in two identified studies. Qualitative research is important for understanding the views and perceptions of Indigenous people (Auger, 2016). It allows insight into the factors that contribute to the co-occurrence of MH conditions and substance use and can better inform future research, policy, and practice. Further investment in Indigenous-led qualitative research is needed to allow for Indigenous people’s knowledge and experience to inform future prevention and treatment-focussed research in this field.

Future Directions

There is a need to increase research examining co-occurring MH conditions and substance use to break down the existing silos in research and treatment and promote integrated care for Indigenous peoples. While consideration of international research can provide some guidance on co-occurring MH conditions and substance use, the unique and diverse Indigenous communities within and between countries require localised data to inform healthcare delivery and policy. Despite several quantitative studies within the US, consideration of prevalence is still needed for all countries to inform needs within distinct communities, specific comorbidities, gender, and age groups. This scoping review intended to provide a surface-level understanding of MH conditions and substance use comorbidity among Indigenous people; however, more in-depth exploration of the research in this area should be considered, such as assessing the quality of quantitative studies and the cultural appropriateness of measures examining MH conditions and substance use. Qualitative research is an important avenue for future work, which will allow for a deeper understanding of strategies for prevention and care that are important for local Indigenous communities. This should be followed by robustly designed experiment studies testing the effectiveness of strategies to prevent and manage co-occurring conditions. Ensuring holistic strategies are developed, accounting for the wider social context in which Indigenous people exist, is also important. This includes consideration of systemic racism and the ongoing, pervasive impact of colonisation.

Limitations

For pragmatic reasons, the search for relevant studies was limited to the English language, which resulted in two studies published in other languages being omitted. This may have included studies that have been published in Indigenous languages. While the search strategy was designed to be broad and capture relevant studies, the diversity of Indigenous groups globally, with hundreds of unique tribes, nations, and clans, may have meant that some studies may have been missed. It was also not possible for the title and abstract screening to be conducted independently by two reviewers in parallel due to staffing constraints, and therefore, this was done sequentially. Nevertheless, all papers returned in the search did undergo independent coding by at least two reviewers. One study did not report outcomes separately for Indigenous and non-Indigenous participants, so some non-Indigenous data was included in the results. However, more than half the participants in this study were Indigenous.

Conclusion

Research commitment and expertise in exploring comorbidity prevention and treatment strategies for Indigenous people is needed. This scoping review highlights several gaps in the field of co-occurring MH conditions and substance use among Indigenous peoples. This includes limited research conducted with Indigenous groups outside of the US and a lack of qualitative or experimental research. Further work is needed to understand the extent and impact of comorbidity among Indigenous people. Future studies should be Indigenous-led with locally informed strategies as well as consultation with Indigenous communities.