Three young adults have suffered severe kidney injuries this month after smoking synthetic cannabinoids, the Canterbury District Health Board says.
Two required treatment in intensive care.
In 2011 the Government banned all existing synthetic cannabis products, including industry-leader Kronic, which had been sold in dairies.
However, new products have skirted the ban by having a slightly altered chemical make-up.
Canterbury medical officer of health Alistair Humphrey said clinicians had growing concerns about the easy accessibility and safety of the drugs, as well as the long term effects they could have on people's physical and mental health.
"There is a potential for severe and permanent toxic injury with smoked or ingested synthetic cannabinoid drugs.
"I'm told that calls to the National Poisons Centre and attendances to emergency departments by patients suffering adverse effects from these drugs are increasing all the time.''
Dr Humphrey said ingredients were not disclosed by manufacturers and he was not aware that the drugs had been properly safety tested.
"Adverse side-effects have included anxiety, vomiting, chest pain and headache. As well as the recent cases of kidney failure other severe side-effects include seizures, psychosis and heart attack.''
Patients typically ranged from 16 to 22 years, with patients as young as 13 years old having reported they are regular users.