A new study shows that persistent use of cannabis by young people can cause an IQ decline in later life.
The study, published on Tuesday, analysed more than 1000 people in Dunedin for almost 40 years.
It found an average loss of eight IQ points over that time for those who began using cannabis before the age of 18.
University of Otago health research director Richie Poulton says memory and attention were also damaged.
Mr Poulton told Radio New Zealand's Morning Report programme it means an average person's IQ would fall from the 50th percentile to the 29th percentile with prolonged cannabis use.
The New Zealand Drug Foundation is urging schools not to be too quick to suspend or expel students caught with drugs.
Executive director Ross Bell says research shows more and more people between the ages of 12 and 13 are turning to drugs, which could affect their intelligence, memory and attention span later in life.
Mr Bell says keeping young people engaged in education is a key factor to prevent them from becoming persistent drug-users.
Some schools too readily opt to suspend or expel students caught with drugs, and more support needs to be offered to keep troubled youth in school, he says.