Use of cannabis, also known as marijuana, during teenage years is linked to lowered intelligence later in life, according to a new study out Monday.

“[We think] that because the brain is undergoing certain maturational changes that make the brain more efficient, when adolescents use cannabis it may disrupt these processes,” says lead researcher Madeline Meier, PhD, of Duke University.

Meier and her team found that those with more persistent use of cannabis had a bigger drop in IQ score — a standardized measure of intelligence — between childhood and adulthood.

“If you start in adulthood, persistent use doesn’t seem to have a big impact on mental ability,” Meier says.

In addition to the IQ scores, friends and relatives of the persistent cannabis users reported more attention and memory problems in everyday life, such as losing focus or forgetting errands and tasks.

The study was performed in New Zealand, where 1,037 children were followed from birth to adulthood. IQ was tested at age 13, before cannabis use began, and again at age 38. New Zealand was selected because cannabis use is more prevalent there than other countries, including the United States, according to Meier.

When the persistent cannabis users stopped, however, the damage to the IQ scores did not reverse. But, Meier says the earlier the teen stops using, the better.

“Our data shows that if you stop, you can prevent further damage from occurring,” she says.

Meier also agrees that further studies need to be done for several reasons. It is unknown whether these findings would correlate to cannabis use in the United States. Also, the adolescents reported their own cannabis use without any testing to confirm that the information was truthful. Lastly, the researchers are unclear as to how much use actually causes the damage.

However, at this point, Meier says the take home is clear.

“Adolescents need to be informed that because their brains are still developing, they are risk of IQ decline [by using cannabis],” she says. “They should delay onset of use until adulthood, and those who began in adolescence should quit.”

Dr. Tyeese Gaines is a physician-journalist with over 10 years of print and broadcast experience, now serving as health editor for theGrio.com. Dr. Ty is also a practicing emergency medicine physician in New Jersey. Follow her on twitter at @doctorty.