The research, the first national study on the impact of the laws, found average treatment rates for new psychoactive substances (NPS) fell by almost 50% in the two years after laws were introduced in 2010.
“Over the two 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,” concluded researchers.
They said during the same time period — 2010 to 2012 — there was no similar trend in relation to the treatment of other drugs, which remained stable.
The study was carried out by consultant psychiatrist Bobby Smyth of the Department of Public Health at Trinity College Dublin, Suzi Lyons of the Health Research Board, and Walter Cullen of the School of Medicine at UCD.
Speaking to the Irish Examiner, Dr Smyth said: “From a public health point of view, to get a 50% reduction is very substantial and significant.
“It supports the view that the action taken by the government then was a reasonable course of action.”
He said that, among young people who never sought treatment previously the drop was greater, at 66%.
Between 2009 and 2010, there was a major increase in the number of headshops, peaking at 102 stores by May 2010, some of them operating 24 hours a day.
The then government extended a legislative ban on NPS, adding 100 substances, in the Misuse of Drugs Act in May 2010.
The Psychoactive Substances Act was introduced in August 2010, banning the sale of any such substances and effectively closed down headshops.
The number of shops fell to 10 by October and, by year’s end, gardaí reported that the remaining shops were no longer selling NPS.
One review claimed the laws were excessive and driven by “moral panic”.
The latest research, published in Drug and Alcohol Review, examined data from the National Drug Treatment Report System.
This documented 58,251 treatment cases over the four years, including 849 where NPS was the problem drug.
Some 756 (89%) of the NPS cases involved young adults, aged 18-34. These cases soared from nine in early 2009 to 122 in mid-2010. Thereafter, it dropped to a low of 38 at the close of 2012.
Dr Smyth said the continued, although significantly reduced, use suggested that users were getting the drug elsewhere — through criminal networks or online.
He said the issue persisted as a particular problem among heroin injectors.