A Great Future For America...

once marijuana is even more widely available.  From August 27, 2012 Reuters:

Researchers from Britain and the United States found that persistent and dependent use of cannabis before the age of 18 may have a so-called neurotoxic effect, but heavy pot use after 18 appears to be less damaging to the brain.

Terrie Moffitt, a psychology and neuroscience professor at King's College London's Institute of Psychiatry, said the scope and length of the study, which involved more than 1,000 people followed up over 40 years, gave its findings added weight.

"It's such a special study that I'm fairly confident cannabis is safe for over-18 brains, but risky for under-18 brains," she said.

Before the age of 18, the brain is still being organized and remodeled to become more efficient and may be more vulnerable to damage from drugs, she added.

Moffitt worked with Madeleine Meier, a post-doctoral researcher at Duke University in the United States, to analyze data on 1,037 New Zealanders who took part in the study. About 96 percent of the original participants stuck with the study from 1972 to today, she said.

Is anyone really surprised to find this out?  Dependent users lost an average of 8 IQ points between 13 and 38.

While 8 IQ points may not sound like a lot on a scale where 100 is the mean, Meier said an IQ drop from 100 to 92 would mean dropping from being in the 50th percentile to being in the 29th.

Higher IQs also correlate with higher levels of education and income, better health and longer lives, she said. "Somebody who loses 8 IQ points as an adolescent may be disadvantaged ... for years to come," she added.

Here's the abstract.  Wired's article about this study notes:

Up for debate is whether the brains of adolescent-onset users recover after quitting marijuana use. Meier’s finding that quitting did not restore mental function to pre-use levels was based on only a small sample of subjects who quit.

 UPDATE: Reading the comments on the Wired article seems to unintentionally prove the point of the study: large numbers of pro-pot comments along the lines of this one:

This study also does not answer the question: Did marijuana lower these people's IQ's? Or do people with low IQ's cope with their situation by smoking marijuana? Cause != effect

Like many of the pro-pot people, he clearly lacked the ability to read the article and understand it:

Researchers gave study participants an IQ test at age 13, before the start of marijuana use, and again at age 38, after some had developed “cannabis dependence” — defined as continued use of the drug in spite of major health, social, and/or legal problems from using it.