The Prevalence of Comorbid Mental Health Difficulties Amongst People With Intellectual Disabilities Who Misuse Substances: A Systematic Review

BACKGROUND: People with intellectual disabilities (ID) are often neglected in substance misuse research, with greater attention paid to the general population more broadly. However, what research does exist suggests substance misuse is problematic for this population whilst also being associated with increased prevalence of comorbid mental health difficulties: a triple diagnosis. This review represents a first attempt to systematically review the existing research on the prevalence of comorbid mental health difficulties amongst people with ID who also misuse substances. METHOD: The study was prospectively registered with PROSPERO (CRD42024484136). Thereafter, a systematic review of the existing literature was conducted across several bibliographic databases (PsycINFO, EMBASE, and MEDLINE). Studies were required to include samples of adults with ID, along with investigating substance misuse and comorbid mental health difficulties. Qualitative studies, reviews and case studies were excluded. A quality assessment was subsequently conducted on included papers. RESULTS: Eight of 3772 retrieved papers were included in this review. Overall prevalence rates of triple diagnosis varied from 8.1% to 78.8%. Schizophrenia, depression and anxiety were reported to be the most prevalent comorbid mental health difficulties amongst people with ID. CONCLUSIONS: People with ID who misuse substances are at greater risk of developing comorbid mental health difficulties, in comparison to the general population, though the precise nature and direction of the relationship require further empirical investigation. A greater focus on early intervention and prevention is required, including greater training of staff in both ID and addictions services in particular, along with focus on more intersectoral working.