A New Zealand scientist is defending a study suggesting teenagers who smoke cannabis do permanent damage to their intellect.
The findings have been challenged by a Norwegian economist who says the researchers failed to take socio-economic factors into account.
Last year, a team of researchers from New Zealand, Britain and the United States analysed IQ tests and rates of cannabis use of about 1000 people involved in the Dunedin Longitudinal Survey.
They found cannabis use before the age of 18 caused an average eight point drop in IQ between the ages of 13 and 38.
Norwegian economist Dr Ole Røgeberg has used computer simulations to show the drop could be accounted for by people from poor backgrounds working in less mentally demanding jobs after they leave school.
However a biostatistician from Auckland University, Thomas Lumley, says data from the longitudinal survey shows people from richer backgrounds also had an IQ decrease if they used cannabis.
He says analysis by the Dunedin group shows IQs do not drop when people do less demanding work.
Professor Lumley says the debate is not relevant to public policy because no one is suggesting cannabis should be legal for under 18s.