Age of Onset and Substance Use Treatment Outcomes Among Emerging Adults in Outpatient Treatment

Background: Studies have shown that substance use typically begins during adolescence, and is a major predictor of developing an addictive disorder later in life. Individuals who use early in life (≤ 14) are 1.5 times more likely to experience alcohol abuse, two times more likely to experience marijuana abuse, and more likely to engage in binge drinking episodes later in life. Early use is also a predictor of association with deviant peers and increased risk of criminal justice involvement. Studies have documented that individuals with more frequent drug/alcohol use are at higher risk of dropping out of treatment (e.g. lower treatment motivation).  However we do not know: if age of onset (AO) (Q1) impacts treatment outcomes (abstinence, alcohol use, binge drinking, cannabis use) or (Q2) if risk factors such as deviant peers and criminal justice association interact with AO or (Q3) if treatment motivation interacts with AO.

Method: The sample consisted of emerging adults (N = 3,550) entering outpatient substance use treatment. Participants were assessed at baseline, 3, 6, and 12-months using the GAIN assessment instrument. We used multi-level growth curve modeling to test the relationship between AO (age of first time getting drunk or using drugs) and multiple treatment outcomes such as days of alcohol use, binge drinking episodes, and days of marijuana use while controlling for deviant peer association, criminal justice involvement, family history of alcohol and drug use, and treatment motivation. We entered higher level interactions to test relationship between treatment motivation, deviant peer risk, and criminal justice involvement with AO.

Results: On average participants were 20.1 years old (SD = 2.25) and mostly male (70.7%). Participants, on average,  reported being 13.5 years old at first age of getting drunk or using other drugs and reported using alcohol on 6.7 days (SD = 14.0), cannabis on 17.2 days (SD = 28.4), and getting drunk on 3.3 (SD= 10.5) days out of the past 90.

Negative bivariate correlations were found at baseline between AO and all treatment outcomes including criminal justice involvement, deviant peer association, and treatment motivation. Further, significant decreases over time were found for alcohol use (b = -.003, 95% CI [.994, .999], p <.01), binge drinking (b = -.003, 95% CI [.994, .999], p <.01), and cannabis use (b = -.008, 95% CI [.988, .996], p<.01) and, in line with Q1, simple slopes analyses revealed a slightly steeper negative slope for late onset users as compared to early onset users (although both were significant). Regarding Q2, no significant interactions were found for AO and deviant peer affiliation or AO and criminal justice involvement. For Q3, significant 3-way interactions were found between AO, treatment motivation and time for alcohol use and binge drinking; simple slopes analysis revealed significant decreases in use for all groups except early onset users with low treatment motivation, who experienced a significant increase in use over time.  

Implications: Our results indicate a need to intervene with early AO individuals with treatments designed to focus on interpersonal motivation (e.g. motivational interviewing).