The study of more than 6000 New Zealanders and Australians found those who used cannabis before the age of 18 ended up achieving less academically those those who did not use the drug.

They were more likely to fail to complete high school, less likely to enter university and less likely to attain a university degree - even when socio-economic status, previous educational achievement and other personal factors were taken into account.

Otago University researcher Professor David Fergusson said the study showed a "robust association" between the early use of cannabis and under-achievement.

The study findings also suggested that early use of cannabis had a greater effect on the achievement of males than females.

The research was based on the combined findings of three Australasian studies, including the University of Otago's long-running Christchurch Health and Development Study, which has tracked more than 900 people from childhood to age 30.

Norml, a group lobbying for cannabis law reform, said the study did not prove cannabis use was the cause of lower achievement.

"A poor-performing student could be attracted to using cannabis, and a bunch of other behaviours that we might not like, but that doesn't mean one is the cause of the other," said Norml spokesman Chris Fowlie.

But his group did not want those under 18 using the drug.

"The current law is not keeping cannabis out of the hands of teenagers. We now have one of the highest rates of teenage cannabis use in the world.

"And that's because we have got this lucrative black market controlled by organised crime."

The illegal nature of cannabis also attracted teenagers as a form of rebellion, Mr Fowlie said.

"We want to have regulated sales so that we can control the age of purchase."