UPDATED 7.44pm: A person is in a critical condition in Christchurch Hospital after using the synthetic drug N-Bomb.

The form of synthetic cannabis is a powerful hallucinogen that sometimes leads to uncontrollable violent behaviour.

Five people have been treated in Christchurch A&E after taking the drug in the last week.

Detective Senior Sergeant Jason Stewart said the last cluster of people needing treatment after using synthetic drugs was six months ago.

He says there's a very real risk of serious harm or fatality... because synthetics users have very little knowledge of what they are actually taking.

"It's a drug that may cause hallucinations, confusion, agitation, sometimes people have become quite violent when they're using it."

N-Bomb is sold as a white powder or in capsules, and has been associated with deaths in Australia and in Asia.