AIMS: To provide public health-related research evidence on types and usage patterns of new psychoactive substances (NPS), developmental pathways into NPS and decision-making factors for, and associated harms of, NPS use.

DESIGN: Three-phase mixed-methods design, including a latent class analysis (LCA) of the longitudinal Belfast Youth Development Study (BYDS), a narrative analysis of interviews with NPS users and a three-step approach manual method modelling using regressions to reveal classes of substance use and their associated predictors and outcomes.
SETTING: Northern Ireland.
PARTICIPANTS: A total of 2039 people who responded to the questions on 'ever use' of the drug variables included at wave 7 (aged 21 years) of the BYDS. Eighty-four narrative interviews with NPS users.
MEASUREMENTS: Categories of drug use identified by LCA. Predictors and outcomes included measures of family, partners, peers, substance use, school, delinquency and mental health.

FINDINGS: A four-class solution provided the best fit for the data: alcohol; alcohol and tobacco; alcohol, tobacco and cannabis; and polydrug (the latter including NPS). The qualitative analysis yielded a taxonomy that distinguished how NPS operate within a wider range of drug repertoires from experimental to problematic.

CONCLUSIONS: In Northern Ireland, new psychoactive substances appear to be a feature of broader polydrug use rather than a standalone class of drug use.