Background Sexual minority groups (SM) have specific substance use patterns that remain widely unknown. We investigated the potential association between substance use among SM adults in the United States (USA) and social inequality, with an additional focus on disparities in unmet need for mental health treatment.

Methods A secondary cross-sectional data analysis was performed using National Survey on Drug Use and Health (NSDUH) data from 2015 to 2017 and including 126,463 individuals with 8,241 identifying as SM. The NSDUH is an annual, nationally representative survey of the civilian, non-institutionalised US population aged twelve or older which estimates the prevalence and associated determinants of substance use and mental illness. Multivariable logistic regression models were used to quantify disparities in substance use among SM and to examine its link with sociodemographic factors and mental health.

Results SM showed higher levels of past-year substance use, misuse of prescription drugs and lifetime chemsex drug use relative to heterosexuals. Gay men showed the highest level of chemsex drug use. Bisexual women exhibited higher levels of use of cocaine, crack, heroin, methamphetamine and misuse of benzodiazepines relative to heterosexual women. Among women, lesbians showed higher odds for use of chemsex drugs and misuse of Oxycontin. Urbanity, being uninsured, and having an unmet need for mental health treatment were associated with significantly higher levels of substance use, including chemsex drug use. SM also experienced significantly higher levels of socioeconomic vulnerability, which were associated with increased odds for drug use.

Conclusion This study is among the first nationally representative samples that analysed the link between sociodemographic factors and unmet need for mental health treatment on substance use among SM. It emphasises the multifactorial aetiology of vulnerability to substance use and highlights the distinct disparities in, and underlying mechanisms for substance use among SM. Higher levels of substance use and misuse observed among SM have public health implications due to the negative health outcomes they may involve in the long-term. Of particular interest is the link established between substance use and mental health issues, considering the lack of tailored approaches aimed at addressing comorbidities and specific health needs of SM, especially given the structural barriers (i.e., stigma, discrimination and criminalisation of substance use) those populations face in accessing health services. Critical gaps in the literature remain and large-scale studies inclusive of SM individuals are needed to establish causal links.