Drug users in South Auckland have been put in hospital after taking what they thought were Ecstasy pills that caused them to react so aggressively they needed to be sedated.
Some users suffered violent seizures and hallucinations that shocked staff at Middlemore Hospital's emergency department.
It is thought all the users took pills known as "red rockets".
Six people were treated last weekend and others continued to arrive up until Friday.
Hospital spokeswoman Lauren Young said emergency department staff immediately noticed the behaviours were not what they would usually see with Ecstasy.
The chemical MDMA gained the street name "Ecstasy" for its euphoric effects but drug dealers often cut their product with other chemicals and some Ecstasy contains no MDMA.
"They were behaviours they thought weren't consistent with Ecstasy ... people had been very aggressive and required lots of settling down and sedation."
Ms Young said there had been at least half a dozen cases last week and another two on Friday night.
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.
It was feared many more pills could be in circulation because it was likely they had been made in large quantities and could now be circulating throughout Auckland's party scene.
The use of Ecstasy, a Class B drug, has been on the rise in New Zealand with most users aged between 25 and 34.
Massey University drugs researcher Dr Chris Wilkins said: "People should be aware they may not be getting MDMA.
"They might get a bunch of chemicals that are entirely different and could include methamphetamine.
"Methamphetamine has got a really bad reputation at the moment as quite a harmful drug. One way to sell methamphetamine is to call it ecstasy and sell it as a powder."By Andrew Koubaridis @A_Koubaridis Email Andrew