Early and Chronic Marijuana Use May Damage Brain Function, Says Study A new study finds that chronic pot smokers perform worse on brain tests.

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.

Marijuana and the Developing Brain

Dr. Donna Seger, executive director of the Tennessee Poison Center and associate professor of Clinical Medicine at Vanderbilt University, said the findings go along with previous research on the effects of drugs on the brain.

"These results are true for most drugs," said Seger. "If you start doing any drug at an early age, you're working with an immature nervous system that is vulnerable to the effects of those drugs."

Marijuana is the most commonly abused illicit substance in the United States. Nearly 26 million Americans reported smoking pot at least once in 2008.

"We're talking about so many variables with this brain function," continued Seger. "Every individual has a different developing nervous system and results of marijuana are likely due to genetic and environmental factors."

Dr. Bennett Leventhal, professor of Child and Adolescent Psychiatry and deputy director of research at the Nathan S. Kline Institute for Psychiatric Research at NYU Medical Center, said that these results are statistically significant, but it will be important to conduct a larger study to confirm the evidence and consider other factors.

"The brain is much more vulnerable to insults like this sort while it is developing and they will need to do a bigger study where stringent controls are in place to include genetics, parental problems with marijuana, and psychopathology in the teen and the parent," said Leventhal. "These results do mean that we should make more effort in educating parents, schools and health officials on the dangers of early onset marijuana exposure."

Pot Up For Debate

And in a time when the legalization of marijuana is hotly debated, Seger said that the results are telling of a growing trend.

"With the lack of legalization, marijuanais getting stronger and stronger all the time," said Seger. "Many people who smoke pot don't want it to be legalized because it wouldn't be as strong once regulated and monitored."

More Research to Come

Gruber agrees that the findings are significant as many states consider legalization of marijuana.

"We have to be clear about getting the message out that marijuana isn't really a benign substance," she said in a statement. "It has a direct effect on executive function. The earlier you begin using it, and the more you use of it, the more significant that effect."

Gruber also said it is unclear whether occasional marijuana use would have the same effect on cognitive function.

"It's impossible to tell whether our results would be the same in moderate smokers," said Gruber. "But there are even more of those people out there, so that will be the next place we go for our research."