Violent and psychotic teenagers high on synthetic cannabis are filling up police cells and hospital emergency departments.

Five people from Bay of Plenty, Canterbury and Nelson have been admitted to hospital with severe kidney failure in recent weeks, after smoking legal synthetic cannabis drugs.

Emergency department doctors were also treating more people with convulsions, vomiting, paranoia and psychosis after having taken the legal highs.

The rise in cases comes as Associate Health Minister Peter Dunne announced a Temporary Class Drug Notice yesterday, banning two more substances found in tested samples of K2 synthetic cannabis.

Police and the Hawke's Bay District Health Board have begun a radio campaign warning the public of the dangers of taking K2.

Hawke's Bay emergency department doctor Brad Sandleback saw at least one patient every shift suffering from the effects of K2 or other synthetic cannabis products.

It was frustrating that people coming in "out of their minds" were sucking up precious hospital resources, Dr Sandleback said.

Security staff were often called upon to watch over agitated and sometimes violent patients.

Doctors could provide "supportive" care only, calming patients and giving them plenty of fluids. There is no antidote.

Doctors were also worried that more people were coming in with "enduring symptoms", National Poison Centre toxicologist Leo Schep said.

Psychotic reactions could last for weeks or even months. Parents were ringing the centre's helpline worried that their sons or daughters were still experiencing psychotic reactions, although they had stopped taking the drug.

Dr Schep said synthetic cannabis products were designed to "lock into" a certain part of the brain.

"Smoking these is like playing Russian roulette."

Eastern district police spokeswoman Kris McGehan hoped the radio advertisements on The Edge and Mai FM would deter young people from buying the drugs.

Police had no jurisdiction over shops selling the legal highs. However, communities around the country were rallying together to try to stamp out their use.

A group of Napier parents, calling themselves the Nanny Brigade, had taken to protesting outside Maraenui shops selling the legal highs.

They wanted to persuade people to take their business elsewhere until the shops stopped stocking them.

In Timaru, dairies agreed to stop selling synthetic cannabinoids after an appeal from mayor Janie Annear.

Mr Dunne's temporary bans, which will come into effect on May 9, bring to 35 the number of substances banned under temporary notices. More than 50 products containing those substances are now off the market.

His Psychoactive Substances Bill, expected to become law in August, will ban all "legal high" products unless their makers can prove they are safe

Dr Sandleback said he had dealt with the effects of legal highs while working in the United States about 10 years ago. "By outlawing it, the problem virtually disappeared."