Teen pot use linked to later declines in IQ
Monday, August 27, 2012
NEW YORK (AP) — 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.
Richie Poulton, a study co-author and professor at the University of Otago in New Zealand, said the message of the research is to stay away from marijuana until adulthood if possible. “For some it’s a legal issue,” he said, “but for me it’s a health issue.”
Pot smoking is relatively common in American teens. The government reported in June that 23 percent of high school students said they’d recently smoked marijuana, making it more popular than cigarettes.
Experts said the new research is an advance because its methods avoid criticisms of some earlier work, which generally did not measure mental performance before marijuana use began.
“I think this is the cleanest study I’ve ever read” that looks for long-term harm from marijuana use, said Dr. Nora Volkow, director of the National Institute on Drug Abuse.
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
The study drew on survey data from more than 1,000 people in New Zealand, everybody born in the town of Dunedin during a year-long span ending in 1973. In addition to IQ tests, they were interviewed five times between ages 18 and 38, including questions related to their marijuana use.
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.
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