Teens may lose IQ points later in life if they smoke marijuana before age 18, according to a study that comes on the heels of a survey showing that the drug's use has risen in this age group for four straight years.
The research, reported Monday in the Proceedings of the National Academy of Sciences, found an average decline of eight points on IQ, or intelligence quotient, tests done at ages 13 and 38 among those who began using marijuana as a teenager. That compared with no decrease in those who used pot later in life, and a slight increase in those who never used it.
Because marijuana is the most frequently used illegal drug in the U.S., looking into on how it changes the brain is important, said study author Madeline Meier, a post-doctoral researcher at Duke University in Durham, North Carolina, who led the study. Daily use among high school seniors is at a 30-year peak, according to a 2011 survey by the National Institute on Drug Abuse in Bethesda, Md.
"What this says is not don't ever do it, but if you do it during this critical period of development, you'll get these long-term negative changes," said Staci Gruber, an assistant professor of psychology at Harvard University in Cambridge, Mass., who wasn't involved in the study. "In almost every case, the subjects who started as adults don't have declines. Those who started as teenagers do."
Tests for intelligence quotient, or IQ, measure a person's capacity to learn, apply knowledge, and use abstract reasoning. The average score is 100.
Although a drop of eight points may not seem like much, going from a score of 100 to 92 drops someone from being in the 50th percentile in intelligence to the 29th, Duke's Meier said.