Patterns of polysubstance use and simultaneous co-use in high risk young adults

•Simultaneous co-use of substances was a powerful indicator of substance use severity.•Lower severity polysubstance use consisted of alcohol and/or cannabis.•High severity polysubstance use consisted of other illicit drugs in addition to alcohol and cannabis.•Antisocial, impulsive, and excitement seeking traits predicted polysubstance use severity.•Severe polysubstance use was associated with high levels of anxiety/neuroticism. Polysubstance use (PSU) is associated with worse prognosis and poorer physical and mental health compared to single substance use. The current study provides information about PSU patterns by examining a diverse range of alcohol/substance use behaviors ranging from low-level experimentation to indicators of severe abuse. In addition, the current study, for the first time, examines how simultaneous co-use of multiple substances cluster with other more commonly studied PSU behaviors. Latent Class Analysis was used to identify patterns of substance use, in a sample of young-adults (n = 2098), using 25 items from the Semi-Structured Assessment for the Genetics of Alcoholism (SSAGA-II) including: items assessing severity of problems with alcohol, cannabis, stimulants, opiates, and sedatives; items assessing # of lifetime uses; items assessing simultaneous co-use of all combinations of substances. Then the association of class membership and age, antisocial and impulsive personality, experience seeking, anxiety, and neuroticism was examined using Multinomial Regression. Fit indices (i.e. AIC, SSABIC, and entropy) and interpretability of classes supported a five-class solution: “Low Problems” (32% of sample), “Alcohol Primary” (11%), “Alcohol and Cannabis” (25%), “Moderate PSU” (23%), and “Severe PSU” (9%). Simultaneous co-use behaviors discriminated between lower and higher severity groups. Externalizing personality constructs robustly predicted membership in the “Moderate” and “Severe” PSU classes compared to the “Alcohol Primary” class. PSU patterns followed an additive pattern of use with lower severity classes using alcohol/cannabis and more severe classes using other illicit substances in addition. Co-use items provided valuable information about PSU severity.