Nov. 16, 2010 (San Diego) -- Early marijuana use appears to take a toll on the brain, according to new research.

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.