A pharmacologist says new batches of synthetic drugs are hundreds of times more powerful than cannabis.
Twenty people have died since July from the drugs, with the chemical AMB-FUBINACA being found in most of them.
Auckland University's Michelle Glass says it's not something humans should be putting in their bodies.
"You need almost 100 times less of it to actually see an effect than you would with THC and cannabis. But at the same time, the effect that you're seeing is much, much greater."
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AMB-FUBINACA was developed by Pfizer, which makes a number of drugs including diazepam, venlafaxine and Viagra. The drug was patented in 2009, but development was abandoned and it was never tested on humans.
A few years later it appeared for sale online as a designer drug.
Police announced yesterday that they'd seized more than $1.5 million-worth of synthetic drug precursors and made a number of arrests.
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Prof Glass says only one thing is known for sure about the drug.
"These weren't designed to go into humans... These are very strong-acting compounds. They are driving the receptor much more strongly than other synthetics we've used in the past."
It works in a similar way to THC - activating the brain's cannabinoid receptors, but much more strongly.
Because there have been no human tests, no one is sure how the drug's effects change when it's mixed with other substances, like alcohol or other drugs users might be taking.
"It's not clear right now how [AMB-FUBINACA causes] the symptoms that are being observed, but we do know that high doses of most synthetic cannabinoids in animals leads to a cataleptic state, which is characterised by a lack of responsiveness, and muscle rigidity which sounds a lot like what people are experiencing," says Prof Glass.
"I don't think anyone knows right now how this is leading to death, but it appears to be related to effects on the heart and seizures.
"There is definitely a need for a lot more research in this area."
There are around 200 known synthetic cannabinoids, ranging in strength from weaker than natural cannabis to the likes of AMB-FUBINACA - dozens of times stronger.
Newshub.
