Early and Chronic Marijuana Use May Damage Brain Function, Says Study

Nov. 15, 2010— -- Chronic pot smokers beware. A new study found that regularly smoking marijuana may lower cognitive function, especially if the person starts smoking before 16 years of age.

Study authors from Harvard-affiliated McLean Hospital wanted to examine the effects of chronic marijuana use on brain function.

In a paper presented today at the annual meeting of the Society for Neuroscience in San Diego, Dr. Staci Gruber, lead author of the small study, reported that study participants who began smoking pot before 16 years of age performed significantly worse on cognitive function tests than both non-smokers and those who became chronic smokers later in life.

Researchers defined chronic marijuana use as smoking pot at least five of the last seven days and a minimum of 3,000 joints in a lifetime. The average age of study participants was 22 years old. The data showed that chronic pot smokers repeated errors more often than the two other groups, even after the authors corrected them. They also had more trouble maintaining a set of rules, suggesting an inability to maintain focus.

"The findings were more striking than I had anticipated," said Gruber. "Although the early onset smokers did the tasks faster than the other group, they made twice as many commission mistakes."

The study authors administered a variety of cognitive tests to 33 chronic marijuana smokers and 26 non-smokers. The tests examined the effects of pot on executive function, which is defined as a collection of brain abilities that are responsible for planning, cognitive flexibility, abstract thinking, and inhibiting inappropriate actions.

It's important to note that Gruber said there was also a significant difference in usage among the early onset smokers and those who started later in life.

"Even before any behavioral differences, we found that those who started smoking before 16 years old tended to smoke twice as often and three times the amount of marijuana in grams than chronic smokers who started smoking later in life," said Gruber.