Factors Associated With Cocaine Use at 17 and 20 Years Old: A Longitudinal Analysis of a Nationally Representative Cohort.

Purpose: Despite growing concerns about trends in cocaine use, there is a shortage of longitudinal research that prospectively examines risk and protective factors associated with cocaine initiation and use in general youth populations. This study addresses this gap.

Methods: Growing Up in Ireland is a nationally representative cohort. Individual, family, and socio-environmental exposures associated with incident past-year cocaine use at ages 17 (N = 5965) and 20 (n = 4549) were assessed with survey-weighted logistic regression using generalised estimating equations. Prevalent past-year cocaine use at 20 (N = 4679) was analysed using generalised estimating equations complemented by gradient-boosted decision trees and Shapley explanations.

Results: 221 (3.7%) self-reported cocaine use at 17 and 1072 (22.9%) at 20. Alcohol use at 14 or younger was associated with eight times the odds of cocaine use at 17 (aOR 8.0, 95% CI 1.7-37.3) and 19 times at 20 (aOR 19.2, 95% CI 8.6-43.2). Peer cannabis use was associated with 7 times the odds of cocaine use at 17 (aOR 7.3, 95% CI 2.9-18.3) and double at 20 (aOR 2.4, 95% CI 1.8-3.2). Growing up in a neighbourhood where substance use was common doubled the odds of cocaine use at 17 (aOR 2.4, 95% CI 1.3-4.4). Shapley explanations revealed individual-specific positive or negative impacts of exposures.

Discussion: Cocaine use among 20-year-olds in Ireland is higher than reported internationally, and increases sharply between the ages of 17 and 20, suggesting a need for interventions targeting this age group. However, associations with early adolescent factors suggest that early interventions may also be important.