Alarm over synthetic drug use in Christchurch is growing after 31 people have been taken to hospital suffering side-effects in the past three weeks, including six requiring intensive care treatment.

Two of these patients remain in intensive care, while another two people - who were not taken to hospital - are believed to have died after also taking the drug recently.

The sharp increase in hospital admissions comes after Canterbury DHB put out a call to the public to be careful on September 21, when 10 people had been taken to Christchurch Hospital's emergency department during the previous 48 hours.

Earlier this month, St John territory manager Craig Downing urged people to stay away from the "poison" and to remember "this is not cannabis in any way shape or form".

"They [synthetic drugs] are extremely dangerous. We don't know what's in them."

Three people were also arrested in Christchurch earlier this month when police discovered a number of drug-induced people in a house.

Detective Senior Sergeant Sarah Illingworth said it was incredibly concerning when officers found a number of people, in one property, who they believed were under the influence of synthetic drugs.

"They were unconscious or close to it. It was a real demonstration of just how dangerous this drug is,'' she said.

"We want the public to know we are working hard to make more arrests like today's to stop the spread of this harmful substance. These drugs are extremely harmful.''

Media outlet Stuff reports police have raided seven properties and charged seven people with supplying psychoactive substances over the past three weeks.