In other words, the brain of a teenage cannabis user appears more susceptible to the effects of the drug than the adult brain, which suggests the bulletproof bravado of the regular teenager smoker is misplaced - and raises, say researchers with interests in the latest study, broader questions about current drug policy.
So just why is the teenage brain vulnerable to a chemical assault from cannabis?
Basically, say scientists, because the youthful brain is still being organised and reshaped to become an efficient adult brain.
Neuroscientist Michelle Glass says that around puberty, the brain goes through a growth spurt, especially the pre-frontal part of the organ which makes high-level decisions.
Associate Professor Glass, head of Auckland University's pharmacology department, said the brain underwent remodelling during these crucial years, so that some connections were strengthened, and others eliminated in a process called synaptic pruning.
Complex brain chemicals control the process of turning the teenage brain into the smarter, better organised adult version. The process has been likened to leaving behind the "noise" of childhood experience to produce the more efficient, streamlined and better-functioning adult brain. New pathways form, and unnecessary ones are discarded as the brain develops in an optimal fashion.
"Simply put," remarked Professor Glass, "if you mess with your brain chemicals then you are going to change the growing and pruning that takes place."
The pull on a joint delivers a cloud of cannabis smoke into the lungs. Carried to the brain in the bloodstream, its active ingredient THC and other chemical components get to work by binding with sites called cannabinoid receptors, a bit like a boat docks at a mooring.
Scientists have established that these receptors react to the body's natural endocannabinoid system, which in turn is associated with memory, appetite, mood and pain-sensation.
From the perspective of the teenage brain, the endocannabinoid system - the chemicals and the receptors - is involved in the refinement of the neurons or cells which process information as the brain passes through the teenage years.
"Taking cannabis during this time can disrupt the natural processes," says Professor Glass. "Once you are an adult the same processes are no longer occurring, so you are particularly vulnerable in puberty."
So can cannabis actually alter the brain structure? Simon Adamson, senior lecturer at Otago University's psychological medicine department in Christchurch, says heavy use can have subtle effects.
Unlike alcohol or solvent abuse, which can lead to severe brain damage, Dr Adamson suggests the findings from the Dunedin study demonstrate that prolonged cannabis use can affect memory, attention span, verbal skills and reasoning.
Put together, Dr Adamson argues that the results suggest concern about heavy cannabis use "should be directed at adolescents".
Ideally, not smoking cannabis at all would be the goal, but short of that, public health policy could be aimed at delaying the start of dope smoking.
Social psychologist Joseph Boden, a researcher with a Christchurch project similar to the Dunedin study, said the evidence suggested the present approach to reducing the risks of drug use was not succeeding.
Cannabis was so widely used that it was seen as normal behaviour, and not deviant or criminal, Dr Boden said.
Though many tried the drug, treatment needed to focus on heavy dope users while evaluating drug education policies.
The Dunedin researchers suggest putting more effort into "delaying the onset of cannabis use by young people", with the implication that if you're going to indulge, then wait until you're an adult.
The Dunedin study findings
The authors followed 1037 children born in Dunedin in 1972 and 1973.
Their report, published in the Proceedings of the National Academy of Sciences, showed that teens who started smoking marijuana before the age of 18 and were diagnosed as being addicted to cannabis by 38 experienced an IQ drop in early adulthood.
But users who began smoking after age 18 - even if they used heavily - did not show a significant decline.
In looking at the relationship between marijuana use and IQ, the researchers took into account controlled factors such as years of education, schizophrenia and use of alcohol or other drugs that might also have an effect on IQ.
