Starting Drinking Before Age 15 Predicted Higher Cannabis Use at Age 20 in Irish Youth
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.
Quick Facts
What This Study 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.
Key Numbers
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 They Did This
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.
Why This Research 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 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.
What This Study Doesn't 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
Questions This Raises
- ?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?
Trust & Context
- Key Stat:
- 24% of Irish 20-year-olds used cannabis, with earlier alcohol initiation predicting higher risk in a dose-response pattern
- Evidence Grade:
- Large nationally representative longitudinal cohort with extensive confounding adjustment; strong design with validated measures
- Study Age:
- Published 2025
- Original Title:
- 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.
- Published In:
- BMC public health, 25(1), 1129 (2025)
- Authors:
- Brennan, Margaret M, Mongan, Deirdre(2), 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
- Database ID:
- RTHC-06114
Evidence Hierarchy
Follows a group of people over time to track how outcomes develop.
What do these levels mean? →Frequently Asked Questions
Does drinking young predict cannabis use later?
Yes. In this Irish study of 4,554 young people, each year later that someone first tried alcohol was associated with progressively lower odds of using cannabis at age 20, even after adjusting for personality, mental health, and social factors.
Is alcohol a gateway to other drugs?
This study found that high-risk drinking at age 17 more than doubled the odds of tobacco (2.1x) and other drug use (2.5x) at 20, and increased continued high-risk alcohol use 11.5-fold, supporting early alcohol as a gateway risk factor.
Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-06114APA
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
MLA
Brennan, Margaret M, et al. "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, 2025. https://doi.org/10.1186/s12889-025-22262-w
RethinkTHC
RethinkTHC Research Database. "Early and risky adolescent alcohol use independently predict..." RTHC-06114. Retrieved from https://rethinkthc.com/research/brennan-2025-early-and-risky-adolescent
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.