The research team led by clinical neuropsychology researcher Krista Lisdahl Medina from University of Cincinnati and Susan Tapert, associate professor of psychiatry at the University of California, San Diego found that chronic, marijuana use during adolescence - a critical period of ongoing brain development, including slower psychomotor speed and poorer complex attention, verbal memory and planning ability.
The impact was seen even after a month of stopping marijuana use.
The study suggests that partial recovery of verbal memory functioning within the first three weeks of adolescent abstinence from marijuana, complex attention skills continue to be affected.
"Not only are their thinking abilities worse, their brain activation to cognitive tasks is abnormal," said Medina.
"The tasks are fairly easy, such as remembering the location of objects, and they may be able to complete the tasks, but what we see is that adolescent marijuana users are using more of their parietal and frontal cortices to complete the tasks.
Their brain is working harder than it should," she added.
Moreover, females might be at increased risk for the neurocognitive consequences of marijuana use during adolescence.
Medina said adolescence is a critical time of brain development and that the findings are yet another warning for adolescents who experiment with drug use.
The study was presented at the 2008 annual meeting of the American Academy of Pediatrics in Boston.