Researchers from Britain and the United States found that persistent and dependent use of cannabis before the age of 18 may have a so-called neurotoxic effect, but heavy pot use after 18 appears to be less damaging to the brain.

Terrie Moffitt, a psychology and neuroscience professor at King's College London's Institute of Psychiatry, said the scope and length of the study, which involved more than 1,000 people followed over 40 years, gave its findings added weight.

"It's such a special study that I'm fairly confident cannabis is safe for over-18 brains, but risky for under-18 brains," she said.

Before the age of 18, the brain is still being organized and remodeled to become more efficient and may be more vulnerable to damage from drugs, she added.

Moffitt worked with Madeleine Meier, a post-doctoral researcher at Duke University in the United States, to analyze data on 1,037 New Zealanders who took part in the study. About 96 percent of the original participants stuck with the study from 1972 to today, she said.

At age 38, all participants were given a battery of psychological tests to assess their memory, processing speed, reasoning and visual processing.

Those who had used pot persistently as teens scored significantly worse in most of the tests. Friends and relatives regularly interviewed as part of the study were more likely to report that the heavy cannabis users had attention and memory problems such as losing focus and forgetting to do tasks.

The researchers also found that people who started using cannabis in adolescence and continued for years afterwards showed an average decline in Intelligence Quotient (IQ) test scores of eight points between the ages of 13 and 38.