Substance Use and Concurrent Disorders: Current Context and the Need for Treatment Integration

Substance use disorders (SUD) are among the most prevalent psychiatric disorders, with estimated lifetime and 12-month prevalence rates of 29.1%/13.9% for alcohol use disorder and 9.9%/3.9% for other drug use disorders.^1,2 SUD prevalence rates are considerably higher among those with other psychiatric disorders (PD). Recent epidemiological data indicate that more than one-third of adults with any PD—and nearly one-half of adults with serious mental illness—meet the criteria for co-occurring SUD. ³ Nonetheless, of those meeting the criteria for concurrent disorders (CD), roughly 40% of adults report no past-year treatment for either SUD or PD and nearly 80% report no SUD treatment. ³ Therefore, the treatment gap for CD remains substantial, with access to SUD treatment being a particularly significant barrier.

The prevalence of CD, currently estimated at roughly 8% for all adults and 15% for young adults, ³ reflects the substantial base rates of both SUD and PD, shared biological and environmental risk factors, and reciprocal associations between substance use and psychiatric symptoms across development. In addition to evidence of shared genetic liability between SUD and other forms of PD, ⁴ common environmental and psychological risk factors—including trauma and socioeconomic disadvantage—increase developmental risk for both SUD and other PD, with these disorders typically manifesting by late adolescence or young adulthood. ⁴ Once initiated, regular substance use can lead to instrumental drug-taking to alleviate negative effects or psychiatric symptoms (i.e., coping-related substance use). ⁵ The maintenance of SUD via negative reinforcement processes—accompanied by allostatic neuroadaptations that ultimately render behaviour change more difficult—is a defining feature of SUD that may also serve to maintain or potentiate other psychiatric symptoms. ⁵ Beyond increasing the risk for numerous medical disorders, long-term exposure to alcohol, tobacco, and other drugs also elevates the risk for neuropsychiatric disorders in later adulthood, including cognitive decline and dementias.^6,7 These considerations argue that, from etiological and epidemiological vantage points, SUD and other PD are in many ways inextricable. By extension, nearly all mental health clinicians will routinely treat those with CD—regardless of whether SUD are formally identified and treated in the context of other psychiatric interventions (often, they are not).

Recent events have reinforced the stark public health consequences of SUD and underscored the need for improved clinical and public health responses to address SUD and CD. The ongoing overdose epidemic—which in 2023 included >8,000 opioid toxicity deaths in Canada and >107,000 drug overdose deaths in the U.S.—is a principal cause of increasing “deaths of despair” (with suicides and alcohol-related liver disease being other leading factors). ⁸ Additionally, the COVID-19 pandemic—which occurred against the backdrop of ongoing increases in overdoses and alcohol-related deaths—introduced acute stress and social isolation, creating a “perfect storm” for worsening SUD-related harms among those at greatest risk, including those with CD. ⁸ Alcohol-related deaths in the U.S. increased by a striking 25% over 1 year (2019 to 2020). ⁹ Though pandemic-related increases in alcohol use and deaths were not universal, rates of alcohol-related disease are projected to further increase, partly as a residual effect of the pandemic. Prior to the pandemic, cannabis legalization in Canada (and increasing access in the U.S.) prompted broad increases in cannabis availability, predicting likely increases in health risks associated with cannabis exposure. Common to each of these events are increases in population-level exposures that would be predicted to yield net increases in drug-related harms, with likely implications for CD. Public health implications of these events are more notable considering limited population-level access to evidence-based treatments for SUD/CD,^3,10 especially relative to other common medical and psychiatric conditions.

The papers in this special issue illustrate recent efforts to characterize or intervene with substance use-related risks in the presence of co-occurring psychiatric symptoms. Posttraumatic stress disorder (PTSD) is associated with marked increases in SUD prevalence, with substance use often occurring in response to PTSD symptoms. The study by Zaso et al. ¹¹ uses innovative ambulatory measurement methods to examine momentary associations between PTSD symptoms, craving, and alcohol use. Results indicate that PTSD symptoms predict momentary alcohol craving among people with stronger momentary motives for negative reinforcement drinking (i.e., stronger coping motives). The study by DeGrace et al., ¹² which also uses remote methods, examines web-based delivery of an experimental task that might help facilitate remote exposure-based therapies for PTSD and co-occurring SUD. Results illustrate that PTSD status predicts higher cannabis cravings during the exposure task. Noting the risks of prenatal cannabis exposure in the context of expanded cannabis access, Nadler et al. ¹³ investigated the relationship of prenatal cannabis, tobacco, and cannabis plus tobacco exposure with childhood externalizing (EXT) symptoms in the Adolescent Brain Cognitive Development dataset. Prenatal exposure to combined cannabis and tobacco was associated with higher EXT symptoms relative to exposure to either drug class alone. Because EXT symptoms are among the most robust developmental predictors of SUD, any causal effects of prenatal exposure on childhood EXT could have implications for offspring SUD risk, highlighting a potentially underrecognized risk of prenatal cannabis exposure.

The well-known associations between EXT symptoms and SUD risk would also predict that clinical samples with SUD/CD will show high rates of EXT-related risk factors. Todesco et al. ¹⁴ examined EXT-related traits, including impulsivity traits and behavioural measures of cognitive control known to predict SUD severity and relapse, in an inpatient CD treatment program. Results showed that EXT-related risk factors were more prominent in CD patients relative to a comparison group. Importantly, no changes in these outcomes were observed following inpatient treatment, raising possible implications for treatment and relapse in CD populations. The next paper from Schütz et al. ¹⁵ reports methodology and baseline data from a multi-site study involving inpatient CD treatment centers in British Columbia and Ontario, which serve as models of integrated CD treatment and research programs. Key findings of this study include the strikingly high overdose rates reported in CD samples: roughly 65% of participants reported lifetime overdoses, with an average of 7–8 overdoses per person. Experiences of lifetime trauma emerged as a robust predictor of overdose risk.

In 1 of 2 randomized treatment trials in this issue, Patel et al. ¹⁶ compared the efficacy of an online cognitive-behavioural SUD intervention to standard group treatment in a tertiary psychiatric setting. Results illustrate the potential advantages and disadvantages of electronic SUD treatments, including the likelihood of greater cost-effectiveness and treatment access, but also the potential for lower efficacy relative to in-person treatments. The study by Schell et al. ¹⁷ illustrates the feasibility of population-level recruitment and retention in brief web-based interventions for substance use and mood symptoms and the possibility that dispositional traits could moderate the efficacy of such interventions. Of note, the latter 2 studies both illustrate the familiar finding of significant prospective reductions in substance use across all intervention groups, reinforcing the axiom that receipt of any SUD intervention is often the most robust predictor of behaviour change. Also notable is that several papers in this issue describe remote assessment and/or intervention procedures symptoms, which are increasingly recognized as critical for expanding treatment access, particularly since the onset of the pandemic.

In the context of recent increases in SUD-related harms, clinicians and stakeholders should seek to ensure that SUD/CD assessment and treatment resources are integrated across the spectrum of mental health intervention and prevention services.^3,10 Historically, SUD and non-SUD services have been largely disjointed, with SUD treatment sometimes required prior to offering other psychiatric interventions. ¹⁰ Best practices dictate that treatments for both SUD and other PD be offered concurrently, or at least at the soonest opportunity. Rather than outsourcing SUD services, integrating evidence-based SUD treatment and referral pathways in clinical and psychiatric settings will optimize patient-centred treatment and continuity of care for those with CD. It is important to note that SUD medications remain vastly underutilized, even in specialty settings. Alcohol and smoking cessation pharmacotherapies can be readily adopted in most medical settings, and policy changes have addressed certain licensing and prescribing barriers to enable more clinicians to prescribe opioid use disorder medications. Clinicians and stakeholders are tasked with ensuring that those with CD can access all possible evidence-based treatments.

Despite recent increases in drug-related harms, a few reasons for optimism can be noted. While rates of heavy drinking and alcohol-related disease have increased for adults, adolescents are one of the few groups in which rates of heavy alcohol consumption have trended downward. If sustained by current and future cohorts, decreasing rates in youth might presage future reductions in heavy drinking and alcohol use disorder prevalence rates. Such a shift could be critical given that rates of alcohol-related liver disease are now at previously unsurpassed levels. Although the COVID-19 pandemic exposed significant risks of stress and isolation for those already at risk for SUD or CD, treatment providers responded by rapidly expanding access to telehealth care, resulting in sustained improvements and efficiencies in many treatment settings. Finally, while progress in identifying new pharmacological treatments for SUD remains painfully slow relative to other areas of medicine, evidence indicates that new treatment options might be on the horizon. ¹⁸ Renewed efforts to integrate evidence-based SUD and CD assessment and treatment resources across the mental health care continuum will help to improve CD treatment access and abate further increases in related harms.