Teen pot use linked to IQ declines

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, presumably before any significant marijuana use, and again at 38. The mental decline between those two ages was seen only in those who started smoking pot regularly before age 18.

Richie Poulton, a study co-author and professor at the University of Otago in New Zealand, said the message 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 is the most popular illegal drug in the world. There were between 119 million and 224 million users between the ages of 15 and 64 as of 2010, the United Nations reported. In the United States, 23 percent of high school students said they'd smoked marijuana recently, making it more popular than cigarettes, the federal government reported in June.

Young people "don't think it's risky," said Staci Gruber, a researcher at the Harvard-affiliated MacLean Hospital in Belmont, Mass. Gruber, who didn't participate in the study, said the idea that marijuana harms the adolescent brain is "something we believe is very likely," and the new finding of IQ declines warrants further investigation.

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, which helped fund the research.

Meier and colleagues reported their work online yesterday 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.