Dr Lambie said many young people trying K2 were upset and surprised by its disturbing effects.
Many had vowed to hospital staff never to try it again.
"They have no idea how toxic it is."
"People think because they can buy it it's harmless . . . the trouble is, I don't know what's in it."
Department staff tried to record blood samples to learn more about the K2's effects, and how long its effects lasted. However, it was "enough trouble" dealing with the rest of the department's workload, and staff did not always have time to take samples.
He could not say how many patients had presented having taken K2, but there were "quite a number".
The department's new observation unit, opened in August, had proved useful for the prolonged watch needed for patients.
All clinicians could do was support patients with fluids, keep them safe from harm, in some cases sedating them with Midazolam.
A spokesman for Associate Health Minister Peter Dunne said results of testing to determine if K2 would be banned would take up to another four weeks.