NEW YORK — Teens who routinely smoke marijuana risk a long-term drop in their IQ, a new study suggests.
The researchers didn’t find the same IQ dip for people who became frequent users of pot after 18. Although experts said the new findings are not definitive, they do fit in with earlier signs that the drug is especially harmful to the developing brain.
“Parents should understand that their adolescents are particularly vulnerable,'” said lead researcher Madeline Meier of Duke University.
Study participants from New Zealand were tested for IQ at age 13, likely before any significant marijuana use, and again at age 38. The mental decline between those two ages was seen only in those who started regularly smoking pot before age 18.
Ken Winters, a psychiatry professor at the University of Minnesota and senior scientist at the Treatment Research Institute in Philadelphia, said the new findings aren’t definitive, but they underscore the importance of studying how marijuana may harm young people. He had no role in the work.
Meier and colleagues reported their work online Monday in the Proceedings of the National Academy of Sciences. It was funded with governmental grants from the United States and Britain, and a foundation in Zurich.
