Smoking Pot might be considered 'cool' in colleges. But, a new study says exposing oneself to marijuana before adulthood may take a hit on intelligence.

According to the international research team that is led by Madeline Meier, a post-doctoral researcher at Duke University, persistent and dependent use of marijuana before 18 years has shown to have lasting adverse effects to a person's intelligence, attention and memory.

But, in people who started smoking pot after 18, researchers did not observe any decline in Intelligent Quotient.

The study was conducted on 1,000 New Zealanders whose IQ were measured using psychological tests at two points in their lives, at the age of 13-less likely to have exposed to pot-and at 38. When the scores were compared, the researchers witnessed 8- point decline in the IQ scores of those who started using pot before 18. Unfortunately, quitting the pot did not seem to reverse the effect.

The age of onset for marijuana use and the brain development is the key variable, according to Meier. She said that the people who did not smoke pot until adulthood with 'fully-formed brains' did not show any apparent dips. However, before 18, teenagers' brains are still being organized and remodelled to become more efficient, she said, and hence more susceptible to harmful effects of drugs.

"Marijuana is not harmless, particularly for adolescents," said Meier, who produced this finding from the long term Dunedin Multidisciplinary Health and Development Study. The study has followed a group of 1,037 children born in 1972-73 in Dunedin, New Zealand from birth to age 38.

The study also involves the researchers Terrie Moffitt and Avshalom Caspi, psychologists who hold dual appointments at Duke and the Institute of Psychiatry at King's College London.

Though, the study talks about the harmful effects of early onset of marijuana use, it does not pinpoint a 'safe age' for persistent use of pot and what dosage level causes damage.

A study by the Partnership at Drugfree.org in May suggested that 1 in 10 American teens smoked pot frequently. As a report by Associated Press shows, US Government reported in June that 23 percent of students said they recently had pot making it more popular than tobacco.

The study results feature online in Proceedings of the National Academy of Sciences (PNAS) Aug.27 onwards.