As New Zealand considers cannabis reform, new evidence has come to light which shows people who smoke strong forms of the drug are at a much greater risk of suffering from a serious mental illness.
Researchers at King's College in London believe the drugs are so potent that people smoking on a daily basis puts people at three times more likely to have an episode of psychosis than those who never use them. The number increases to five times more likely for people using high strength doses of the drug.
More potent forms of cannabis are dominating the market in London and Amsterdam, the BBC reports.
Users of the particularly potent forms have described the episodes as both frightening and debilitating.
A former user told the BBC of spirits coming through the walls, floors and ceiling, paranoid episodes and being too scared to leave his flat.
The report, published in the Medical Journal Lancet Psychiatry, said the results are also particularly harmful for younger people.