A new study shows that getting synthetic cannabis off the shelves has virtually stopped the flow of users needing mental health care.
In 2013, the Government passed the Psychoactive Substances Act, banning all but 41 so-called legal highs from sale.
Otago University chair of psychiatry Paul Glue, who is also a consultant psychiatrist at the Southern DHB, led a study into the impact of the law change, published today in the New Zealand Medical Journal.
Professor Glue said, before the law change, emergency departments saw young people with psychosis, severe mood problems, aggression, depression, anxiety and suicidal tendencies as a result of taking legal highs.
He said that cost New Zealand about $25 million a year.
"In early 2013, we had a large number of young people turning up who needed admission to hospital related to smoking synthetic cannabis... and so it was obviously a real concern in terms of patients' health and safety, and really as a public health problem as well."
Professor Glue said the difference in demand from legal high users for mental health services before and after the law change was stark.
"In the middle of 2013, the Psychoactive Substances BIll came along, and that approximately halved the number of products that were available for sale, but more importantly it reduced the number of places where synthetic cannabis could be sold from," he said.
"We saw a 50 percent reduction in attendances at ED [emergency department] or EPS [emergency psychiatric service], but the presentation was exactly the same, exactly the same kind of demographic, these were primarily young men who had histories of mental illness."
Last year, following high profile campaigns and media coverage about the impact of synthetic highs on individuals and communities, the law was further tightened, removing all products from the shelves.
Under the law, a product has to pass a testing regime and be sold with a licence, and no companies have so far applied for or received a licence.
Professor Glue said, after this change, the number of people needing care after taking legal highs almost disappeared altogether.
"People had gone out and bought lots of products, and then smoked it, and once it was gone, the harm stopped."