Substance use, and substance use disorders (SUD), are major contributors to global burden of disease among youth. They constitute an important risk factor for other disorders, including mental disorders. This thesis seeks to examine methods to reduce the harm associated with youth substance use, looking at the impact of treatment on the individual adolescent with a SUD and at legislative measures operating at the population level. While treatment models for SUD have become more liberal and harm reduction orientated in recent decades, legislation remains conservative and prohibitionist.

The archetypal harm reduction treatment is opiate substitution treatment (OST) used with heroin dependence. The evidence base for OST in adolescents is sparse. Outcome of OST was examined. It emerged that OST delivers early reductions in heroin use, which continue to improve significantly from month three to month twelve of treatment. Evidence of improved psychological wellbeing is also demonstrated by adolescents on OST. In order to explore the impact of conservative legislative measures on harm related to substance use, a quasi-experimental approach was undertaken to explore changes which occurred in Ireland before, during and after the arrival of a vast network of head shops selling new psychoactive substances (NPS). Evidence is presented indicating that the expansion of head shops coincided with increased NPS addiction episodes among both adolescents and young adults. There was also evidence of increased drug related psychiatric admissions. All of these harms began to diminish within months of the closure of the head shops.

Overall, these findings lend support to the position of providing tolerant and responsive treatment which does not demand abstinence for the small subset of youth who develop a SUD, while simultaneously maintaining an intolerant and conservative approach to prevention of substance use at the wider population level.