If you smoke marijuana, you will permanently lose eight IQ points as chronic THC exposure hastens the age-related loss of hippocampal neurons. The resulting impairments in neural connectivity will degrade memory, learning and impulse control, eventually leading to an increased likelihood of becoming a heroin addict.
At least that's what the federal government says on the National Institute on Drug Abuse's website in a post titled “What are marijuana’s long-term effects on the brain?“.
Scares about marijuana’s long-term effects are as old as prohibition itself. The problem for the scaremongers is that there are plenty of older pot smokers actively debunking all the scares. Whoopi Goldberg is in her fifties and chatting it up on The ViewCheech Marin is in his sixties tearing through a game of Celebrity Jeopardy!  Tommy Chong’s in his seventies and "Dancing with the Stars."  Willie Nelson is in his eighties and still "On the Road Again."  While they’re not rocket scientists, I don’t think anyone would call these celebrities stupid.
The eight IQ points nonsense derives from a study that came out of Duke University in summer 2012.  Prohibitionists like Kevin Sabet, facing the prospect of two legal marijuana states, pounced on the study's conclusion and have beaten that talking point into the ground ever since.
However, the problems with the study were numerous.  It only found the decline among the heaviest consumers of marijuana, which was only 38 people out of 1,073 in the study.  It found slight increases in IQ for moderate consumers.  A follow-up study found other socioeconomic factors explained the IQ drop just as reasonably as the marijuana use.  It was hardly the slam-dunk that the drug warriors wanted it to be.
Last fall, another study came out from University of London.  It tracked 2,612 kids born in 1991 and 1992 and checked IQ scores at age 8 and age 15.  The scientists found absolutely “no relationship between cannabis use and lower IQ at age 15.”  Even the heaviest pot smokers didn’t lose IQ points.
They did find, however, that alcohol consumption was predictive of losing IQ points.  Ain’t it funny how you don’t see any TV ads about that?  Why doesn’t Kevin Sabet ever bring up that fact?
Likewise, there are many studies prohibitionists like to seize on to justify their stereotyped perception of marijuana consumers as dullards.  Most are studies that look at results in rats and extrapolate the results to humans.  Others take a look at brain scans of marijuana consumers and interpret the results.  But when scientists examine actual humans who consume marijuana, they find little to no differences in cognitive function.
One of the first to uncover this truth was Dr. Carl Hart, the Columbia neuroscientist and author of “High Price: A Neuroscientist’s Journey of Self-Discovery That Challenges Everything You Know About Drugs and Society.”  In 2001, he published research showing that regular marijuana consumers showed no mental decline while high.  Regular consumption, he found, led to a tolerance to marijuana’s impairing effects - the infrequent toker who gets blazed might have more trouble solving the crossword puzzle, for example, but not the regular toker.
By 2011, other researchers began following up on this hypothesis. One study followed almost 2,000 Australian adults for eight years.  They found little to no decline in learning and memory, and what decline they found reversed itself after a short period of abstinence.
As for that gateway to heroin, it looks like National Institute on Drug Abuse hasn’t read the 1999 report from the Institute of Medicine, which found that marijuana “does not appear to be a gateway drug to the extent that it is the cause or even that it is the most significant predictor of serious drug abuse.”  You’d think these Institutes would communicate better.
But the simplest proof you need is to just talk to an older pot smoker.  You’ll find they are not any smarter or stupider than any other person you meet, but their stories are better, and they’re likely to share a bowl with you as long as you listen.
(Photo of normal brain PET Scan courtesy of National Institutes of Health)
http://www.hightimes.com/read/reports-your-brain-marijuana-just-fine