Marijuana is the world’s most widely used illicit drug. But smoke too many joints too soon, and you might lose IQ points: A new study suggests extensive marijuana use starting as a teenager could lead to cognitive decline.

In a study of 1,037 New Zealanders followed from birth to age 38, people who began using marijuana as adolescents and used it extensively for years saw their IQs drop by about eight points. What’s more, among adolescent-onset users, quitting the drug did not reverse the mental deficits.

“Marijuana is not harmless, and particularly not for adolescents,” said study author Madeline Meier, a psychologist at Duke University. Meier’s findings were reported Aug. 27 in the Proceedings of the National Academy of Sciences.

Researchers gave study participants an IQ test at age 13, before the start of marijuana use, and again at age 38, after some had developed “cannabis dependence” — defined as continued use of the drug in spite of major health, social, and/or legal problems from using it.

To get a sense of how significant a decline of eight IQ points is, consider this: Having an average IQ of 100 puts you in the 50th percentile for intelligence, whereas an IQ of 92 slides you down to the 29th percentile.