Young adults who began smoking marijuana before age 16 performed worse on cognitive tests compared to those who began smoking after age 16, says researcher Staci Ann Gruber, PhD, of Harvard Medical School and McLean Hospital, Boston. She presented her findings at a news conference Monday at Neuroscience 2010, the annual meeting of the Society for Neuroscience in San Diego.
Although several studies have already found that earlier use of marijuana can lead to cognitive problems, the new study directly compares the differences between early and late-onset users, Gruber says.
Gruber found that early-onset smokers, once she evaluated them as young adults, also smoked more -- and more often. "Early-onset smokers smoked twice as often and nearly three times as much," she says.
Early-onset smokers averaged 25 smokes a week, while the late-onset users averaged 12. Early-onset users averaged 14 grams a week, while the late-onset users averaged under 6 grams a week.
Focusing on Early Marijuana Use
For the study, Gruber evaluated 35 chronic marijuana smokers and compared them to 29 healthy people, including men and women. Twenty of the smokers started before age 16 and 15 started after age 16.
At the time of the evaluation, their average age was 22.
Gruber says 16 was chosen as the cutoff age between early- and late-onset smoking ''because that's when teens tend to [begin to] use drugs."
She gave them a card-sorting task commonly used to evaluate cognitive flexibility, known as the Wisconsin Card Sort Task. People are shown cards with different shapes on them. The shapes are in different colors, in different numbers, and in different forms. The person tested is told to sort the cards, without being told how, and then is told if the sorting is correct.
Next, the person giving the test changes the rule on how to sort, but doesn't tell the participant, who must figure out the new rule.
How the test taker responds to the rule change and how quickly he grasps the new rule reflects his cognitive flexibility.
The test is considered a measure of executive function, which involves such tasks as planning, abstract thinking, and moderating one's social behavior.
The healthy comparison group performed better on several measures, compared to the marijuana smokers, and the early-onset smokers did worse than the late-onset smokers. The early-onset smokers ''kept not getting it," Gruber says. Despite being told their sorting was wrong after the rules change, the early-onset users kept making repeated errors. The early users also had more problems maintaining a set of rules.
For one category, for instance, early-onset smokers made 14.2 mistakes, compared to 6.4 by the late-onset smokers.
How might these cognitive problems translate to everyday life? Gruber says the early-onset smokers may have trouble with abstract thinking, for instance, or may say something socially inappropriate.
Early Marijuana Use: Perspective
The new research adds to previous studies finding other harmful brain effects, says Frances Jensen, MD, a professor of neurology at Harvard Medical School who moderated the news conference detailing Gruber's findings.
She speculates that the marijuana exposure ''is somehow modifying the way that part of the brain [the prefrontal cortex, involved in executive function tasks] is developing," she tells WebMD. "That area is one of the last parts of the brain to develop fully, she says, so the teen brain is especially vulnerable.
''Currently, it looks irreversible," she says of the damage incurred by early-onset smoking of marijuana.
This study was presented at a medical conference. The findings should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.