The study found that those who frequently used marijuana before the age of 18 had an eight-point decline in their IQ — what’s interesting, is the decline wasn’t seen in those who started smoking in early adulthood or later in life.
The findings suggest that long-term marijuana use can cause long-term harm to some thinking abilities, such as intelligence, memory, and attention span, and that teens are uniquely vulnerable, researchers say. “Many people today, especially young people, believe that marijuana is risk free, but this research tells us that this is not the case,” says Temple University professor of psychology Laurence Steinberg, PhD, who was not involved with the study.
For the study, Duke University postdoctoral researcher Madeline H. Meier, PhD, and colleagues compared IQ scores before the participants had ever tried marijuana (age 13) and those at age 38, when many had experience with the drug. Study participants who used marijuana in their teens and continued to use the drug for many years had an average decline in IQ of eight points by the time they reached 38 years old.
Meier tells WebMD that marijuana use may be particularly neurotoxic to teens because adolescence is a time when the brain is developing.
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