The persistent, dependent use of marijuana before age 18 has
been shown to cause lasting harm to a person's intelligence,
attention and memory, according to an international research
team. Among a long-range study cohort of more than 1,000 New
Zealanders, individuals who started using cannabis in
adolescence and used it for years afterward showed an average
decline in IQ of 8 points when their age 13 and age 38 IQ tests
were compared. Quitting pot did not appear to reverse the loss
either, said lead researcher Madeline Meier, a post-doctoral
researcher at Duke University. The results appear online Aug. 27
in PNAS. The key variable in this is the age of onset for
marijuana use and the brain's development, Meier said. Study
subjects who didn't take up pot until they were adults with
fully-formed brains did not show similar mental declines. Before
age 18, however, the brain is still being organized and
remodeled to become more efficient, she said, and may be more
vulnerable to damage from drugs. "Marijuana is no