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.
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 is the most popular illegal drug in the world, with somewhere between 119 million and 224 million users between the ages of 15 and 64 as of 2010, the United Nations reported. Within the United States, 23 percent of high school students said they'd recently smoked marijuana, 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 new work, 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.