A comprehensive study of 4,695 young people in Ireland has revealed that 23.2% of 20-year-olds engage in polysubstance use among young adults. This research, published in September 2025, represents one of the largest population-based studies examining contemporary substance use patterns amongst young adults.

The Growing Up in Ireland study tracked participants from age 9 to 20, providing unprecedented insights into how substance use develops over time. The findings demonstrate clear escalation patterns that begin with early alcohol consumption and progress to multiple substance use.

Four Distinct Categories of Polysubstance Use Among Young Adults

Researchers identified four distinct patterns through statistical analysis of substance use behaviours:

Limited Use Group (33.8%): These individuals showed low-risk alcohol consumption with minimal use of other substances. This group had a 71% probability of low-risk alcohol use and near-zero probabilities for other substances.

Alcohol, Tobacco and Cannabis Group (43.0%): This largest group demonstrated combined use of three primary substances. They showed 87% probability of low- to increasing-risk alcohol use, 92% probability of tobacco use, and 94% probability of cannabis use.

Polysubstance Use Group (16.2%): These young adults polysubstance use patterns included five or more substances. The group showed 79% probability of increasing- to high-risk alcohol use, 71% probability of current tobacco use, 63% probability of cannabis use, 76% probability of cocaine use, and 66% probability of ecstasy use.

Heavy Polysubstance Use Group (7.0%): The highest-risk category demonstrated 95% probability of problematic alcohol use, 83% probability of daily tobacco use, 78% probability of cannabis use, 98% probability of cocaine use, 99% probability of ecstasy use, and 90% probability of ketamine use.

Critical Statistics on Risk Factors

The research identified specific measurable risk factors that dramatically increase the likelihood of polysubstance use among young adults:

Early Alcohol Initiation: Young people who began drinking at age 14 or younger faced substantially higher risks. Those who delayed alcohol consumption until age 17 showed 91% reduced odds of heavy polysubstance use compared to early initiators.

Peer Influence: Having friends who used cannabis at age 17 created the strongest risk factor. This increased odds of heavy polysubstance use by 1,350% (13.5 times higher) compared to those without cannabis-using peers.

Family History: Young adults with family members who had alcohol or drug problems showed 380% increased odds (3.8 times higher) of developing heavy polysubstance use patterns.

Gender Differences: Males demonstrated 265% increased odds (2.65 times higher) of heavy polysubstance use compared to females.

Living Arrangements: Those living away from parents showed 230% increased odds (2.3 times higher) of heavy polysubstance use.

Geographic Location: Living in the Dublin region increased odds of heavy polysubstance use by 194% (1.94 times higher) compared to other areas.

Substance-Specific Usage Patterns

The study measured specific substance use rates amongst 20-year-olds:

• Alcohol: 50.0% classified as increasing-risk use, 8.6% as high-risk, and 4.9% as potentially dependent
• Tobacco: 38.0% used tobacco occasionally or daily
• Cannabis: 24.3% used cannabis occasionally or weekly
• Cocaine: 23.0% reported past-year use
• Ecstasy: 18.0% reported past-year use
• E-cigarettes: 14.0% used occasionally or daily
• Ketamine: 12.0% reported past-year use

Early Alcohol Use as a Gateway Predictor

The data reveals a clear dose-response relationship between age of alcohol initiation and later polysubstance use among young adults:

• Age 14 or younger: Highest risk group (reference category)
• Age 15: 28% reduction in heavy polysubstance use risk
• Age 16: 89% reduction in heavy polysubstance use risk
• Age 17: 96% reduction in heavy polysubstance use risk
• Age 18 or older/no alcohol: 99.8% reduction in heavy polysubstance use risk

Critically, 90% of study participants had initiated alcohol use before the legal age of 18, highlighting widespread underage drinking.

International Context and Comparison Data

This Irish study found higher polysubstance use rates compared to previous international research:

• Denmark: 26% of males and 9% of females in polysubstance classes
• Northern Ireland: 10% in polysubstance classes
• Australia: 10.0% to 13.5% polysubstance use prevalence
• United States: 6% to 15.4% in various studies
• Ireland (current study): 23.2% in polysubstance classes

The higher rates may reflect temporal changes in substance use patterns, as many previous studies used data from 2001-2011, whilst this research examined 2018-2019 behaviours.

Treatment-Seeking Population Statistics

Supporting the research findings, Ireland’s treatment data shows concerning trends:

• 2023: 13,104 individuals sought drug treatment (highest annual number recorded)
• Polysubstance use: 58.9% of treatment seekers reported multiple substance use
• Common combinations: Cocaine and alcohol most frequently used together, followed by cocaine and cannabis, then opioids and cocaine

Prevention Through Evidence-Based Intervention

The statistical evidence supports several targeted prevention approaches:

Delay Alcohol Initiation: Each year of delayed alcohol use significantly reduces polysubstance use risk. Prevention programmes should focus intensively on ages 13-16.

Address Peer Influence: Given the 1,350% increased risk from cannabis-using friends, social skills programmes and peer support networks become critical intervention points.

Family-Based Approaches: With 380% increased risk from family substance use history, prevention efforts should include family therapy and education components.

Geographic Targeting: Urban areas require enhanced prevention resources given the 194% increased risk in metropolitan regions.

Long-Term Health and Social Implications

Young adults polysubstance use carries measurable risks supported by extensive research literature:

• Increased mental health problems
• Higher overdose risks from unpredictable substance interactions
• Greater likelihood of risky sexual behaviours
• Elevated risks of developing substance dependence disorders
• Increased probability of legal and social consequences

The combination of substances creates compounding effects that exceed risks from individual substance use, making prevention efforts particularly crucial for this population.

Source: Online Library