Teenagers who smoke cannabis regularly could be permanently damaging the development of their brain and are likely to end up with significantly lower IQ scores than teenagers who do not use the illicit drug, a major study has found.

People who started smoking cannabis as adolescents were found at the age of 38 to be still suffering from a drug habit they had started more than 20 years earlier, scientists said.

The findings will help to dispel the common belief among teenagers that cannabis is a harmless drug and will lend weight to calls for more to be done to prevent cannabis use among teenagers, the researchers said.

The study suggests that weekly cannabis use before the age of 18 results in an average decline in IQ score of eight points, which is enough to move someone of average intelligence into a category that is well below average.

Scientists said that the study is the first to show that cannabis use in adolescence – but not cannabis use that begins in adulthood – can cause a significant long-term decline in IQ that does not appear to be reversible when people stop using cannabis.

The researchers believe this is evidence that cannabis can interfere with the development of the adolescent brain, which continues to undergo neural growth and "rewiring" during early teenage years.

"Quitting or reducing cannabis use did not appear to fully restore intellectual functioning," said Madeline Meier, of Duke University in North Carolina. She was the lead author of the study published in Proceedings of the National Academy of Sciences. " IQ decline could not be explained by alcohol or other drug use or by reduced years of education among persistent cannabis users… marijuana is not harmless, particularly for adolescents," Dr Meier said. "Somebody who loses eight IQ points as an adolescent may be disadvantaged compared to their same-age peers for years to come." The study used data gathered from a cohort of 1,037 children born in 1972-73 in Dunedin, New Zealand. IQ tests carried out when they were 13 were compared with IQ tests completed when they were 38.

Five per cent of the cohort said they had started persistent cannabis use – defined as weekly sessions – before the age of 18.

These individuals were compared with the rest of the group both in terms of IQ and other possible interfering factors, as well evidence based on detailed interviews with friends and family.

"The people who used pot persistently as teens scored significantly worse on most of the tests," Dr Meier said. "Friends and relatives routinely interviewed as part of the study were more likely to report that the persistent cannabis users had attention and memory problems such as losing focus and forgetting to do tasks."

An average loss of eight IQ points out of an average score of 100 would mean a drop of IQ to 92, which would be enough to move someone from a group shared by 50 per cent of the population to a lower group shared by just 29 per cent of the population.

"Research has shown that IQ is a strong determinant of a person's access to a college education, their lifelong total income, their access to a good job, their performance on the job, their tendency to develop heart disease, Alzheimer's disease and even early death," Dr Meier said.

"Individuals who lose eight points in their teens may be disadvantaged, relative to their same-age peers, in most of the important aspects of life and for years to come."

Laurence Steinberg, a psychologist at Temple University in Philadelphia, said the study is the first to distinguish between cognitive problems that might result from using cannabis in adolescence, from those that existed prior to the cannabis use.