POT SMOKING CHANGES TEENS' BRAINS, STUDY SHOWS

Smoking marijuana regularly before the age of 16 causes changes in
the brain that can
impair a young person's ability to focus, learn from mistakes and
think abstractly, according to a Harvard study.

On brain scans, the youngest pot smokers showed activation in regions
of the brain that was not seen in those who started smoking after age
16, suggesting early exposure to marijuana causes neural changes,
researchers at Harvard-affiliated McLean Hospital found. Early and
habitual users performed more poorly on tests of cognitive functions,
including mental flexibility.

Research on how marijuana changes a developing brain is important
because marijuana is the most frequently used illegal drug in the
U.S., said study author Staci Gruber, director of the cognitive and
clinical neuroimaging core at McLean Hospital in Belmont, Mass. Almost
16 percent of eighth-graders have tried marijuana, and that number
rises to 42 percent by 12th grade, a 2009 study from the National
Institute on Drug Abuse found.

Chronic, early users of marijuana "make repetitive incorrect responses
despite the fact I'm telling them they're wrong," said Gruber, who is
also an assistant professor of psychology at Harvard University.
"That's called 'cognitive inflexibility,' and you see it in babies."

The research, presented Monday at the Society for Neuroscience's
meeting in San Diego, also found that the group that started earlier
smoked more pot more often than those who started later. People who
began smoking before age 16 had 25.1 smokes a week, compared with 12.1
in those who began later, the study showed. The early-onset group
smoked almost three times as many grams a week, Gruber said.