INTRODUCTION AND AIMS: New psychoactive substances (NPS) have hedonic effects that may lead to dependence. Headshops selling NPS increased in number in Ireland from late 2009. Legislation was enacted in May and August of 2010 that caused their closure. It is unknown whether such events impact the rate of NPS use disorders.

DESIGNS AND METHODS: We conducted a population-based study using the Irish national database of episodes of addiction treatment between 2009 and 2012. We examined trends in the rate of NPS-related treatment episodes among young adults. Joinpoint trend analysis software was used to identify significant changes in trend.

RESULTS: Of the 31 284 episodes of addiction treatment commenced by adults aged 18 to 34 years, 756 (2.4%) were NPS related. In 2012, the 12-month moving average rate had fallen 48% from its peak in 2010, from 9.0/100 000 to 4.7/100 000. Joinpoint analysis indicated that the rate of NPS related episodes increased by 218% (95% confidence interval 86 to 445, P = 0.001) every 4 months until the first third of 2010. From that point, the rate declined by 9.8% (95% confidence interval -14.1 to -5.4, P = 0.001) per 4-month period. There was no significant trend change in the rate of non-NPS related treatment episodes.

DISCUSSION AND CONCLUSIONS: Over the 2 years after the enactment of prohibition-styled legislation targeting NPS and headshops, the rate of NPS related addiction treatment episodes among young adults declined progressively and substantially. We found no coinciding trend change in the rate of episodes linked to other drug groups.