In a nationally representative Irish cohort of 4,554 young people, earlier age of first alcohol use and risky adolescent drinking independently predicted cannabis and other drug use at age 20 in a dose-response pattern.
Public health officials, adolescent health researchers, substance use prevention designers, parents
24% of Irish 20-year-olds used cannabis, with earlier alcohol initiation predicting higher risk in a dose-response pattern
What the researchers found
Older age at first alcohol use was associated with dose-response reductions in cannabis use odds at age 20 (relative to initiation at 14 or younger). Adolescents with high-risk drinking at 17 had 2.1x odds of tobacco use and 2.5x odds of other drug use at 20. 27% reported first alcohol use at 14 or younger; by 20, 24% used cannabis.
Why it matters
This large longitudinal study with extensive confounding adjustment provides strong evidence that early alcohol use is a gateway risk factor for cannabis and other drug use, independent of personality, mental health, and social factors. It supports the case for delaying adolescent alcohol initiation as a broad substance use prevention strategy.
The numbers in context
4,554 participants (49.8% female); 27% first drink at age 14 or younger; at age 20: 14% high-risk alcohol, 38% tobacco, 24% cannabis, 28% other drugs; high-risk alcohol at 17 predicted 11.5x odds of continued high-risk alcohol, 2.1x tobacco, 2.5x other drugs at 20; dose-response with age of first drink
How the study worked
Nationally representative Growing Up in Ireland cohort (n=4,554, born 1997-1998). Survey-weighted logistic regression examined age at first drink and risky drinking at 17 as independent predictors of substance use at 20, adjusted for sex, academic ability, personality, psychological factors, SES, and family/peer substance use.
What this study cannot tell us
Observational design cannot prove causation despite extensive adjustment, Irish context may not generalize to all cultures, self-reported substance use, age 20 follow-up may not capture later patterns, attrition between waves
How to read the evidence
Large nationally representative longitudinal cohort with extensive confounding adjustment; strong design with validated measures
When this study was published
Published 2025
The bigger picture
The gateway drug debate has focused on cannabis as a gateway to harder drugs, but this study positions early alcohol use as the more significant gateway, predicting cannabis, tobacco, and other drug use in a dose-response pattern that persists after extensive adjustment.
Questions still open
- Would raising the effective age of first alcohol exposure reduce cannabis use at the population level? Are the same patterns present in countries with different drinking cultures? Does the dose-response relationship extend beyond age 20?
Common questions
Does drinking young predict cannabis use later?
Is alcohol a gateway to other drugs?
Read the original research
Early and risky adolescent alcohol use independently predict alcohol, tobacco, cannabis and other drug use in early adulthood in Ireland: a longitudinal analysis of a nationally representative cohort.
BMC public health, 25(1), 1129
Citation
Brennan, Margaret M; Mongan, Deirdre; Doyle, Anne; Millar, Seán R; Cavallaro, Massimo; Zgaga, Lina; Smyth, Bobby P; Nixon, Elizabeth; Ivers, Jo-Hanna; Galvin, Brian; Walsh, Cathal; McCrory, Cathal; McCarthy, Noel D. (2025). Early and risky adolescent alcohol use independently predict alcohol, tobacco, cannabis and other drug use in early adulthood in Ireland: a longitudinal analysis of a nationally representative cohort.. BMC public health, 25(1), 1129. https://doi.org/10.1186/s12889-025-22262-w
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