AGGRESSION: Christchurch’s Youth and Cultural Development centre manager Anni Watkin says K2, a synthetic cannabis product, has become a major problem only weeks after appearing in Christchurch.

A legal synthetic cannabis drug that has caused major problems for police and health authorities in the South Island.

K2, an R18 product sold in dairies, video stores and adult stores for $20 a packet, has a growing reputation for making users feel aggressive, paranoid and even suicidal within a few hours of the initial high.

One heavy cannabis user in Christchurch has likened its effect to being closer to class A drug methamphetamine, or P.

Sergeant Kelvin Giddens said K2 emerged about a month ago and had quickly become "a major problem" among young people.

Some dairies were selling up to 50 packets a week, he said.

He had spoken to a heavy cannabis user who used K2 and afterwards experienced suicidal thoughts for the first time.

"It's in our interests to get it classified or put on the list not to be sold," Giddens said.

In Dunedin, incidents involving young people acting aggressively after taking K2 became such a problem that police asked dairy owners to stop selling it.

Christchurch Hospital emergency specialist Paul Gee said the number of patients admitted to the emergency department after using synthetic cannabis-type drugs like K2 was growing. The side effects appeared worse than products of the past, including those now banned, he said.

Agitation, confusion, paranoia, seizures and violent behaviour could last days to months, Gee said.

One patient had been admitted to intensive care with severe toxicity. Others had to be given sedatives or subdued by police before being admitted to hospital.

Patients were mostly young adults but included some school pupils, he said.

Christchurch police were contacted by ambulance staff after a 17-year-old had a seizure after he smoked a synthetic cannabis product last month.

There had also been recent cases of synthetic cannabinoid-related crime.

Last week, two hooded robbers entered the Kosco Asian supermarket in Main North Rd, Belfast, said they had a knife and ran off with about five packets of K2. They have not been arrested.

In September, a man allegedly indecently assaulted a 12-year-old girl after giving her a pipe filled with synthetic cannabinoid. The girl was unable to walk and was described as "out of it", a police spokesman said.

A 44-year-old man faces charges of stupefying and indecent assault on a child.

Christchurch's Youth and Cultural Development centre manager Anni Watkin said K2 had caught on quickly among their young people in the past two weeks and had "a massive effect" on their behaviour.

"They reach a high over a period of one to three hours, then the problem is when they plummet," she said.

They became aggressive, emotional, depressed, confused, panicky and even suicidal.

It took only one young person to try it and put a message on Facebook, she said.

"It's just a replacement for [Kronic]. They [producers] will continue to produce and replace whatever is taken off the market," she said.

Christchurch Rehabilitation Services clinical director David Stoner said he had seen cases where K2 brought on relapses of pre-existing mental health issues.

On the West Coast, even first-time admissions to psychiatric services were related to K2.

"People have a misconception [K2] is safe because it is sold next to the chewing gum," he said.

A Health Ministry spokesman said the ministry was "concerned about K2 and has ongoing dealings with the police on this matter".

Legislation banning the sale of all psychoactive substances unless tested and approved by a regulator is expected to take effect in the middle of next year.

Kronic was banned, along with 43 other synthetic cannabis products, in August last year.

Associate Health Minister Peter Dunne said K2 was being tested by the ministry because of concerns it was causing serious behavioural problems.

Detective Senior Sergeant Brian Archer, head of the Christchurch police drug squad, said that if K2 problems escalated in the city, police would develop a strategy to target it.