At age 18, 52 participants indicated they had become dependent on marijuana, meaning that they continued to use it despite its causing significant health, social or legal problems. Ninety-two others reported dependence starting at a later age.
Researchers compared their IQ scores at age 13 to the score at age 38 and found a drop only in those who had become dependent by 18.
Those deemed dependent in three or more surveys had a drop averaging 8 points. For a person of average intelligence, an 8-point drop would mean ranking higher than only 29 percent of the population rather than 50 percent, the researchers said.
Among participants who'd been dependent at 18 and in at least one later survey, quitting didn't remove the problem. IQ declines showed up even if they'd largely or entirely quit using pot at age 38, analysis showed.