MONDAY, Aug. 27 (HealthDay News) -- Teens who start smoking marijuana regularly experience what appear to be permanent declines in their IQs and other aspects of mental function, new research finds.
The study included information on more than 1,000 people born in New Zealand in 1972-1973. Participants took IQ and other mental functioning tests at age 13 -- before any had started smoking marijuana -- and then again at age 38.
Every few years, starting at age 18, participants were also asked about their use of marijuana and assessed for marijuana dependence. Marijuana dependence is defined as someone who feels they need to smoke more and more marijuana to get the same effect, who has tried to quit but can't or who keeps using even though the habit is causing them problems, such as with their health, family, work or school.
About 5 percent reported using marijuana more than once a week before age 18 or were considered marijuana-dependent at one or more points during the study.
Those who started smoking marijuana heavily as teens -- meaning at least once or week -- or were diagnosed with marijuana dependence before age 18 and who continued to smoke into adulthood showed an average 8-point drop in their IQs by age 38.
People who started using marijuana heavily later -- not as teenagers -- also experienced an IQ drop, but it wasn't as severe, the investigators found.
"The findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects," said study author Madeline Meier, a postdoctoral research associate in psychology and neurology at Duke University.
For those who had started as teens, quitting or cutting back on marijuana later didn't seem to help much, according to the study. Even if they stopped, intellectual functioning never came back to the previous level
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