Given that medical cannabis is becoming quite prevalent throughout the USA, what are the current views of long-term and permanent damage caused to the brain and mental performance? Is it true that marijuana might shrink the brain or is neurotoxic?
Scientists have learned a great deal about how THC acts in the brain to produce its many effects. When someone smokes marijuana, THC rapidly passes from the lungs into the bloodstream, which carries the chemical to the brain and other organs throughout the body.
THC acts upon specific sites in the brain, called cannabinoid receptors, kicking off a series of cellular reactions that ultimately lead to the "high" that users experience when they smoke marijuana. Some brain areas have many cannabinoid receptors; others have few or none. The highest density of cannabinoid receptors are found in parts of the brain that influence pleasure, memory, thinking, concentrating, sensory and time perception, and coordinated movement.1
Not surprisingly, marijuana intoxication can cause distorted perceptions, impaired coordination, difficulty with thinking and problem solving, and problems with learning and memory. Research has shown that, in chronic users, marijuana's adverse impact on learning and memory can last for days or weeks after the acute effects of the drug wear off.2 As a result, someone who smokes marijuana every day may be functioning at a suboptimal intellectual level all of the time.
Research into the effects of long-term cannabis use on the structure of the brain has yielded inconsistent results. It may be that the effects are too subtle for reliable detection by current techniques. A similar challenge arises in studies of the effects of chronic marijuana use on brain function. Brain imaging studies in chronic users tend to show some consistent alterations, but their connection to impaired cognitive functioning is far from clear. This uncertainty may stem from confounding factors such as other drug use, residual drug effects, or withdrawal symptoms in long-term chronic users. From the N.I.D.A.
In February 2000 researchers in Madrid announced they had destroyed incurable brain tumors in rats by injecting them with THC, the active ingredient in cannabis. The study was later published in the journal Nature Cancer Review.
Chances are that you have never heard of this study, the same as you likely never heard of a previous similar study because there has been a virtual news blackout as well as a concerted government effort to suppress such stories and studies for over thirty years.
The study by Manuel Guzman of Madrid Spain found that cannabinoids, the active components of marijuana, inhibited tumor growth in laboratory animals by modulating key cell-signaling pathways and thus causing direct growth arrest and death of tumor cells. The study also found that cannabinoids inhibited angiogenesis and that cannabinoids were usually well tolerated and did not produce the generalized toxic effects of conventional chemotherapies.
According to neurologist Dr. Ethan Russo, the Guzman study was very important because cancer cells become immortalized and fail to heed normal signals to turn off growth and die on cue. In addition, the other ways that tumors grow is by sending out signals to promote angiogenesis, the growth of new blood vessels. Cannabinoids turn off these signals as well.
Normally, any story that even suggests the possibility of a new treatment for cancer is greeted with headlines about a "cancer cure" - however remote or improbable it might be. However, if marijuana is involved, don't expect any coverage from mainstream media.
News coverage of the Madrid discovery has been virtually nonexistent in this country. The news broke quietly on Feb. 29, 2000 with a story that ran once on the UPI wire about the Nature Medicine article. The New York Times, The Washington Post, and Los Angeles Times all ignored the story, even though its newsworthiness would seem indisputable: a benign substance occurring in nature destroys deadly brain tumors.
The previous study which indicated that marijuana could be effective against cancer was conducted in 1974. In that study, researchers at the Medical College of Virginia, who had been funded by the National Institutes of Health to find evidence that marijuana damages the immune system, found instead that THC slowed the growth of three kinds of cancer in mice - lung and breast cancer, and virus-induced leukemia.
The DEA quickly shut down the Virginia study and all further cannabis/tumor research, according to Jack Herer, who reported on the events in his book, "The Emperor Wears No Clothes". In 1976, President Gerald Ford ended all public research on cannabis and granted exclusive research rights to major pharmaceutical companies, who unsuccessfully attempted to develop synthetic forms of THC that would deliver the medical benefits without the "high."
In 1983, the Reagan/Bush Administration attempted to persuade American universities and researchers to destroy all 1966-76 cannabis research work, including compendiums in libraries, reported Herer. He stated, "We know that large amounts of information have since disappeared."
On March 29, 2001, the San Antonio Current printed a story by Raymond Cushing titled, "POT SHRINKS TUMORS; GOVERNMENT KNEW IN '74" which detailed government and media suppression of news about marijuana cancer benefits. Cushing noted in his article that it was hard to believe that the knowledge that cannabis can be used to fight cancer has been suppressed for almost thirty years and aptly concluded his article by saying:
"Millions of people have died horrible deaths and in many cases, families exhausted their savings on dangerous, toxic and expensive drugs. Now we are just beginning to realize that while marijuana has never killed anyone, marijuana prohibition has killed millions."
“New Study Explains How Pot Kills Cancer Cells,” Americans for Safe Access, November 10th, 2003, safeaccessnow.org
“Pot Shrinks Tumors - Government Knew in '74,” Raymond Cushing,Source: San Antonio Current, drugpolicycentral.com
Wow. This is a great summary. I really appreciate the time and effort, not to mention the professional penmanship needed to maintain some level of integrity. Cheers, and thanks for the info.
During my recent jaunt of research, I have dug up a few (unfortunately) negative reports. Personally, I am biased, and would hope that these negative reports are flawed. Such a report was produced in 2008  where the research is focused on measuring the sizes of various regions of the human brain.
Ultimately, the researchers found: "Cannabis users had bilaterally reduced hippocampal and amygdala volumes"
I wonder if the researchers had given the subjects longer to detox, if the results would have been closer to the control? I assume though that the brain recovers, after use, but I am unsure of this.
A conflicting report in 2005  suggests that "[...] no significant adjusted differences in volumes of gray matter, white matter, cerebrospinal fluid, or left and right hippocampus. Moreover, hippocampal volume [...] These findings are consistent with recent literature suggesting that cannabis use is not associated with structural changes within the brain as a whole or the hippocampus in particular." The researchers still say that "The effects of cannabis smoking on the morphology of the hippocampus are still unclear"
So it seems to me that no one is really sure of the brain morphology in humans.
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