Polysubstance use – the consumption of multiple psychoactive substances within a short time frame – is increasingly recognised as a major public health issue. A new nationally representative study of young adults in Ireland, based on the Growing Up in Ireland (GUI) study cohort, sheds important light on the prevalence and risk factors associated with this behaviour.1

In this research, which has been published in the journal Addiction, the authors followed more than 4,600 individuals from childhood into early adulthood, analysing their substance use at age 20 years. Using latent class analysis, the study identified four distinct patterns of use. Approximately one-third (33.8%) of participants were classified as ‘limited users’, with low-risk alcohol consumption and little involvement with other substances. The largest group (43.0%) fell into the ‘alcohol, tobacco, and cannabis’ class, reflecting high rates of use of these three substances but relatively low engagement with other drugs. Almost one in four young adults belonged to a polysubstance class: around 16.2% were engaged in regular use of alcohol, tobacco, cannabis, cocaine, and ecstasy, and 7.0% exhibited ‘heavy polysubstance use’, characterised by frequent and diverse drug consumption, including ketamine and other substances. These individuals displayed the highest likelihood of high-risk or dependent alcohol use and repeated use of multiple illicit drugs.

The analysis also identified a number of individual, familial, and social factors associated with higher-risk substance use. Male participants were significantly more likely to belong to polysubstance classes than females were. Early initiation of alcohol use, particularly drinking before the age of 15 years, was strongly linked with later polysubstance use. Family background and social influences played an important role: participants who reported parental alcohol or drug problems were at greater risk of polysubstance use, as were those whose parents had lower levels of education. Having friends who used cannabis at the age of 17 years was associated with a dramatic increase in the odds of belonging to a polysubstance class by the age of 20 years. Living outside the parental home and residing in Dublin were also linked with heavier patterns of use, likely reflecting increased independence and greater drug availability in urban areas.

The findings position Ireland at the higher end of international comparisons. While studies in the United States of America and Australia typically report polysubstance use prevalence rates of between 6% and 15%, the Irish figure of 23.2% is substantially greater. Cultural factors, including the normalisation of heavy drinking and the growing availability of cocaine in Ireland, may help explain this difference.

From a policy perspective, the study underscores the need for integrated approaches that address multiple substances simultaneously, rather than focusing on individual drugs. Prevention strategies aimed at delaying alcohol use initiation, supporting families, and reducing peer influence are likely to be the most effective. The study authors suggest that the implementation of Ireland’s Public Health (Alcohol) Act 2018, which includes measures such as minimum unit pricing and restrictions on marketing, represents a step in the right direction. However, additional harm reduction and education initiatives will be needed in order to address the complex realities of polysubstance use.

In conclusion, this research reveals that almost one-quarter of Irish young adults engage in polysubstance use by the age of 20 years. Addressing this issue will require sustained public health investment, robust prevention strategies, and continued monitoring as this cohort progresses further into adulthood.

1   Brennan MM, Mongan D, Doyle A, et al. (2025) Polysubstance use in early adulthood and associated factors in the Republic of Ireland: An analysis of a nationally representative cohort. Addiction, 121(1): 150–162. Available from: https://doi.org/10.1111/add.70182