Experts say synthetic cannabis, blamed for at least seven recent deaths in Auckland, can be up to 50 times stronger than the real thing.
Police have issued a desperate warning for people to stay away from the drug, with the city's emergency services attending two dozen callouts a day.
While there has been speculation the drugs are being laced with more potent chemicals, Wellington Hospital emergency medicine specialist Dr Paul Quigley says synthetic cannabis is harmful enough on its own.
He says synthetic drugs lack ingredients found in natural cannabis that temper the effect of THC, the psychoactive ingredient that gives cannabis its high.
"It is usually balanced by the presence of cannabidiol and other active chemicals in natural cannabis that mediates its effects. It is cannabinol that has the majority of the medicinal actions of cannabis.
"Not only is synthetic cannabis 100 percent THC-active, the chemical is far more potent than naturally occurring THC, having up to 50 times the effect of natural THC."
Even the average synthetic marijuana has between five and 10 times the THC strength, he says.
"Even a single smoke of synthetic is the equivalent of up to 15 normal joints. This is why the effect is so very different and so very dangerous."
- End poverty and people won't smoke synthetics - campaigner
- Trio in synthetic cannabis stupor filmed in Auckland playground
Side-effects, even at low doses, include epileptic seizures, heart palpitations, anxiety, insomnia, kidney impairment and rapid addiction.
"Because synthetic cannabis is so much more potent than natural cannabis we were also seeing the effects related to substance abuse and addiction developing rapidly," says Dr Quigley.
"Users would need to smoke regularly every three to four hours, even waking during the night to smoke and would exhibit signs of withdrawal on cessation."
Synthetic cannabis products were banned in 2014, but are now reportedly one of the easiest drugs to find on the streets.
There are no reported deaths as a direct result of overdosing on natural cannabis.