The father of a man who's in a critical condition in Christchurch Hospital after taking the synthetic drug N-bomb says he's shocked, and is trying to come to grips with what has happened.

Michael has been in hospital since Wednesday and was admitted alongside five others who'd also taken the drug.

The 20-year-old is on life support and is suffering from multiple organ failure.

His father Murray says he didn't know his son was dabbling in recreational drug use, and didn't even know the drug existed.

He says he's concerned about how many young people know where to get it and how cheap it is.

The drug itself is a form of synthetic cannabis.

Police say the drug is unpredictable and highly dangerous.