A batch of Ecstasy pills that caused users to react so aggressively they had to be sedated may have contained a new drug that has caused similar reactions in the United States and Britain.
Staff at Middlemore Hospital's emergency department were shocked by the violent seizures and hallucinations in people thought to have taken pills known as "red rockets".
They reported users to be so aggressive that some required sedation - behaviour not normally consistent with Ecstasy.
Six people were treated last weekend. At least three of the cases were from Pukekohe, where police have issued a warning for anyone who has the red rocket pills not to take them.
Because police have not seized any of the pills, tests cannot be done to determine their content.
But National Poisons Centre toxicologist Dr Leo Schep , said symptoms such as hallucinations were similar to those caused by a new synthetic stimulant drug sometimes sold as "bath salts".
"I'm not saying it definitely is, but based on the symptoms that have been reported ... it strongly suggests the possibility it's related to bath salts."
The drugs are named after traditional bath salts because they usually come in a powder or crystal form.
They contain man-made chemicals such as mephedrone and methylenedioxypyrovalerone (MDPV), related to khat, an organic stimulant found in Arab and East African countries.
Mephedrone became popular in Britain as a legal substitute for cocaine, but use there has dropped since it was banned in April 2010.
MDPV was illegal in New Zealand as it is a controlled drug analogue of the Class B2 controlled drug pyrovalerone, Dr Schep said.
This month the US Drug Enforcement Agency temporarily banned the synthetic chemicals in the drugs after health professionals said large doses of sedatives failed to calm some users.
Incidents included a man who climbed a flagpole and jumped into traffic, a man who broke into a monastery and stabbed a priest, and a woman who scratched herself raw because she thought there was something under her skin, the New York Times reported.
Massey University drugs researcher Dr Chris Wilkins said mephedrone had been mentioned by New Zealand drug users in Massey's annual Illicit Drug Monitoring System study.
"We didn't get a lot of people who said they'd used it for the first time, but it certainly appears that it's here."
Dr Wilkins said the red rockets could contain mephedrone because Ecstasy pills were now cut with a range of synthetic stimulant drugs.
A global shortage of the precursors to MDMA, the traditional ingredient in Ecstasy pills, had meant many pills sold in New Zealand as Ecstasy contained no MDMA at all.
"A range of other substances have taken its place, and one is mephedrone."
* Related to khat, an organic stimulant found in Arab and East African countries.
* Sometimes sold as "bath salts" due to its appearance.
* Stimulant has been linked to episodes of violence and paranoia.
* Active chemical most commonly associated with the drug Ecstasy.
* Synthesised at the turn of the 20th century, becoming popular as a "dance drug" in the 1980s.
* Ecstasy today may not contain any MDMA due to world shortages.