How Marijuana Really Affects the Brain
Although there’s a lot of buzz about marijuana being nonaddictive, the evidence is stacking up that people can — and do — become dependent on the drug. A study released earlier this year, for example, found that 40 percent of marijuana users in an outpatient treatment program showed signs of withdrawal, a classic indicator of addiction. Now, new research in the journal PNAS sheds light on how lighting up changes the brain — and potentially primes people for withdrawal.
It’s long been known that exposure to THC, the primary psychoactive ingredient in pot, can lead to changes in the brain. Problem is, different studies have shown different structural alterations, making it difficult to pinpoint exactly how the stuff affects people mentally. That’s why a group of researchers decided to use three different magnetic resonance imaging (MRI) techniques to examine the brains of 48 chronic marijuana users and 62 nonusers, while also assessing IQ and negative life consequences of pot smoking.
The most obvious difference: The people who regularly toked up had less volume in the orbitofrontal gyri. This brain region is part of the orbitofrontal cortex, “one of the primary areas within the reward system, which is basically a network of brain regions implicated in the addiction process,” said study author Francesca Filbey, an addiction researcher at the Center for BrainHealth at the University of Texas at Dallas. “More specifically, the orbitofrontal cortex is important for decision-making. This is the area of the brain that would learn something is good for us or bad for us.”
So why does pot cause shrinkage in this area? Simple: The orbitofrontal cortex is highly concentrated with cannabinoid receptors, the places in your brain where THC binds. As a result, it’s much more vulnerable to the effects of a chronic flood of the substance. In animal studies, “the number of those receptors decreased as a result of THC exposure as a way to regain balance in that system,” Filbey told Yahoo Health. “So too much THC basically leads to lower numbers of those receptors in the brain.”
https://www.yahoo.com/health/how-marijuana-really-affects-the-brain-102531080862.html
I wasn't sure whether to post this here or the science forum but choose here even though I don't think it should be illegal. More addictive & harmful substances are already legal & I feel it is important to know how things put into your body affect you.
After the addiction, it becomes less clear. Longer use probably would cause shrinkage or change other areas of the brain. Though I'm not sure what they mean by "withdrawal". I personally felt somewhat irritable & maybe a little anxiety but not much more than that (I usually go 6-12 months apart between a 1-2 weeks of regular use -- once in a blue moon kind a thing)
Link to study
http://www.pnas.org/content/early/2014/11/05/1415297111.full.pdf+html
= new reply since forum marked as read
It was meant towards the article. 
Don't let anyone get you down.
Some people only want to hear what they want to hear and will attack anything that doesn't fit their world view.
The strength and potency has grown over time, I'm glad I quit years ago, it made me paranoid and want to get un-high.
Medical uses, great. Recreational, whatever floats your boat.
But I think it has addictive qualities and it definitely impairs one's ability to drive and do a few other things, so it's not entirely benign.
IMO, one can't be both because our bodies and our minds have evolved to very sophisticated and sensitive states.
The same is true for tobacco and alcohol use and of particular concern for young bodies and minds: These are foreign chemical compounds that the body doesn't need and doesn't want, and they INTERFERE with normal development.
Fairly benign, but by no means are any of these without effect. The degree to which pot is physically or psychologically addictive is going to vary by individual and continue to be studied for generations to come.
From the American Psychological Association:
Her talk, part of the “Friends of NIDA” series co-sponsored by APA, gave congressional aides and policymakers the latest research findings on marijuana’s harms and benefits. One-third of Americans have tried marijuana at least once, Volkow said, making it the most commonly used illegal drug in the United States — and a prime target for research. NIDA estimates that about 9 percent of frequent marijuana users are dependent on the drug.
“People take marijuana for the same reason they take other drugs: They make you feel good,” Volkow said.
That good feeling is tied to the dopamine-based reward system in the brain’s nucleus accumbens region. Compounds in marijuana bind to the brain’s cannabinoid receptors, triggering dopamine release and resulting in a high. Long-term use of marijuana not only increases the amount of the drug that users need to reach the same high, it also inhibits the brain’s natural cannabinoids. As a result, over time users feel dysphoric and “off” if they haven’t recently taken marijuana. Marijuana also targets and interferes with cannabinoid receptors in areas of the brain crucial to a number of cognitive functions, especially the cerebellum (movement), hippocampus (memory) and amygdala (emotional control).
Interfering with those cognitive processes is particularly dangerous for young people’s developing brains, Volkow said, and there’s evidence to suggest using marijuana at an early age can have lifetime consequences. Twin studies show that people exposed to marijuana as young teens are more likely to become dependent on other drugs, such as cocaine and painkillers.
http://www.apa.org/monitor/2011/05/marijuana.aspx
This is a good read, too:
Four lines of evidence all substantiate that marijuana is addictive: basic neuroscience, animal studies, clinical reports of human experience with marijuana, and epidemiology. Data from these four areas of scientific research corroborate each other and interweave fluidly to dispel the myth that marijuana is not addictive.
In order to place the following information in its proper perspective, medicine's current understanding of how marijuana interacts with the brain needs to be outlined. Scientific research has discovered an extensive system of nerves within the brain that communicate with each other using the same basic chemistry found in marijuana. The THC (tetrahydrocannabinol) and similar molecules in marijuana are able to affect the brain by mimicking our natural neurotransmitters and flooding receptor sites with stimulation. All the cannabinoid-based areas of the brain are subsequently activated beyond normal physiological levels by using marijuana.
While we are only beginning to unravel the pervasive role the endocannabinoid (i.e., the brain's naturally occurring THC-like molecules) system plays in overall brain function, Raphael Mechoulam, one of the most important pioneers in cannabinoid research, has declared that "The cannabinoid receptors are found in higher concentrations than any other receptor in the brain... and the endocannabinoid system acts essentially in just about every physiological system that people have looked into, so it appears to be a very central system."
2)
http://www.csam-asam.org/marijuanas-addictive-potential-healthcare-professionals
"Some people only want to hear what they want to hear "
But by no means is it benign and harmless to use, especially for young people who are still developing physically and emotionally.
I'm of the opinion that prolonged and chronic pot use severely slows development of social and cognitive skills.
Sometimes the caricatures are accurate...
Anyway, the preponderance of evidence suggests that it's best to keep a clear head and steer clear of mind-altering substances altogether, if you want to live a healthy and natural life. For medical purposes, of course, it's worth the risk.
Four lines of evidence all substantiate that marijuana is addictive: basic neuroscience, animal studies, clinical reports of human experience with marijuana, and epidemiology. Data from these four areas of scientific research corroborate each other and interweave fluidly to dispel the myth that marijuana is not addictive.
In order to place the following information in its proper perspective, medicine's current understanding of how marijuana interacts with the brain needs to be outlined. Scientific research has discovered an extensive system of nerves within the brain that communicate with each other using the same basic chemistry found in marijuana. The THC (tetrahydrocannabinol) and similar molecules in marijuana are able to affect the brain by mimicking our natural neurotransmitters and flooding receptor sites with stimulation. All the cannabinoid-based areas of the brain are subsequently activated beyond normal physiological levels by using marijuana.
While we are only beginning to unravel the pervasive role the endocannabinoid (i.e., the brain's naturally occurring THC-like molecules) system plays in overall brain function, Raphael Mechoulam, one of the most important pioneers in cannabinoid research, has declared that "The cannabinoid receptors are found in higher concentrations than any other receptor in the brain... and the endocannabinoid system acts essentially in just about every physiological system that people have looked into, so it appears to be a very central system."
2)
http://www.csam-asam.org/marijuanas-addictive-potential-healthcare-professionals