Several studies find further evidence of detrimental effects caused by marijuana smoking

Since this wave of legalization of use of recreational marijuana is spreading gradually but surely to more and more states in the United States, more and more studies funded by medical institutions and organizations are reporting on the very real dangers of frequent marijuana use. I am going to highlight only a few of the studies published on the subject in various medical journals in the past few years.

Journal of Neuroscience (April 16, 2014)

This study conducted by researchers from Harvard University and Chicago's Northwestern Medicine group, using a small sample of 40 individuals between the ages of 18-25 has shown that smoking cannabis ( also known as marijuana) once or twice a week can lead to major changes in brain areas associated with emotions and motivation.

In an article on this study, "Smoking cannabis could change the part of the brain dealing with motivation, according to one new study" published in The Independent ( April 16th 2014), health reporter Charlie Cooper writes;

The researchers used neuroimaging techniques to analyze the brains of cannabis users and non-users.

They found that the nucleus accumbens was unusually large in the cannabis users, while the amygdala also had noticeable abnormalities.
Anne Blood, assistant professor of psychiatry at Harvard Medical School said that the areas affected were "core, fundamental structures of the brain".
"They form the basis for how you assess positive and negative features about things in the environment and make decisions," she said.
The severity of abnormalities in these regions of the brain was directly related to the number of joints a person smoked per week, according to the study, published in the Journal of Neuroscience on Wednesday. The more joints a person smoked, the more abnormal the shape, volume and density of the brain regions, but the effect was noticeable even in those who smoked once or twice a week.
However, experts in the UK said that the study group was small and that more research was needed over a longer timescale to establish whether cannabis smoking caused the unusual brain features, or whether people with such brain features were more likely to smoke cannabis in the first place.
Around one million people aged between 16 and 24 use cannabis in the UK per year, according to the charity DrugScope. Its use has been reported to cause anxiety and paranoia in some users and in rarer cases may be a trigger for underlying mental health problems.
Dr Michael Bloomfield, clinical research fellow at the UK's Medical Research Council (MRC), said that the study added to the MRC's own research which found that heavy cannabis use in adolescence is associated with changes in chemical connections in the brain.

Schizophrenia Bulletin ( December 16, 2013)

Yet another study conducted by Northwestern Medicine and mainly funded by The National Institute of Mental Health and National Institute of Drug Abuse, has shown that heavy use of marijuana (daily for 3 years) in teen years can lead to abnormal changes in the brain structure related to memory. The teens in the study performed poorly on memory tasks.
According to lead study author Matthew Smith, an assistant research professor in psychiatry and behavioral sciences at Northwestern University Feinberg School of Medicine, " The study links the chronic use of marijuana to these concerning brain abnormalities that appear to last for at least a few years after people stop using it, with the movement to decriminalize marijuana, we need more research to understand its effect on the brain."
Chronic use of marijuana may contribute to changes in brain structure that are associated with having schizophrenia, the Northwestern research shows. Of the 15 marijuana smokers who had schizophrenia in the study, 90 percent started heavily using the drug before they developed the mental disorder. Marijuana abuse has been linked to developing schizophrenia in prior research.

As reported in 'Heavy marijuana users have abnormal brain structure and poor memory'

by Eureka Alert online science news service;

"The abuse of popular street drugs, such as marijuana, may have dangerous implications for young people who are developing or have developed mental disorders," said co-senior study author John Csernansky, M.D., chair of psychiatry and behavioral sciences at Northwestern University Feinberg School of Medicine and Northwestern Memorial Hospital. "This paper is among the first to reveal that the use of marijuana may contribute to the changes in brain structure that have been associated with having schizophrenia."
Chronic marijuana use could augment the underlying disease process associated with schizophrenia, Smith noted. "If someone has a family history of schizophrenia, they are increasing their risk of developing schizophrenia if they abuse marijuana," he said.
While chronic marijuana smokers and chronic marijuana smokers with schizophrenia both had brain changes related to the drug, subjects with the mental disorder had greater deterioration in the thalamus. That structure is the communication hub of the brain and is critical for learning, memory and communications between brain regions. The brain regions examined in this study also affect motivation, which is already notably impaired in people with schizophrenia.
"A tremendous amount of addiction research has focused on brain regions traditionally connected with reward/aversion function, and thus motivation," noted co-senior study author Hans Breiter, M.D., professor of psychiatry and behavioral sciences and director of the Warren Wright Adolescent Center at Feinberg and Northwestern Memorial. "This study very nicely extends the set of regions of concern to include those involved with working memory and higher level cognitive functions necessary for how well you organize your life and can work in society."


Journal of Chemical Research in Toxicology (May 18, 2009)

The study Dr. Michael Bloomfield mentions was conducted by  Leicester University’s Rajinder Singh, Jatinderpal Sandhu, Balvinder Kaur, Tina Juren, William P. Steward, Dan Segerback and Peter B. Farmer from the Cancer Biomarkers and Prevention Group, Department of Cancer Studies and Molecular Medicine and Karolinska Institute, Sweden. This research was funded by MRC, European Union Network of Excellence (ECNIS) and Cancer Research UK. The findings were published in the Journal of Chemical Research in Toxicology.
In this case researchers found "convincing evidence" that cannabis smoke damages DNA in ways that could potentially increase the risk of cancer development in humans.  

Lead author Dr Singh said:
“There have been many studies on the toxicity of tobacco smoke. It is known that tobacco smoke contains 4000 chemicals of which 60 are classed as carcinogens. Cannabis in contrast has not been so well studied. It is less combustible than tobacco and is often mixed with tobacco in use. Cannabis smoke contains 400 compounds including 60 cannabinoids. However, because of its lower combustibility it contains 50% more carcinogenic polycyclic aromatic hydrocarbons including naphthalene, benzanthracene, and benzopyrene, than tobacco smoke.” 
The authors added: “It is well known that toxic substances in tobacco smoke can damage DNA and increase the risk of lung and other cancers. Scientists were unsure though whether cannabis smoke would have the same effect. Our research has focused on the toxicity of acetaldehyde, which is present in both tobacco and cannabis.”
The researchers add that the ability of cannabis smoke to damage DNA has significant human health implications especially as users tend to inhale more deeply than cigarette smokers, which increases respiratory burden.
"These results provide evidence for the DNA damaging potential of cannabis smoke," the researchers conclude, "implying that the consumption of cannabis cigarettes may be detrimental to human health with the possibility to initiate cancer development."

Although in each case marijuana use supporters will argue the sample sizes are too small or the study was biased, the results of each of these researches were very real. If anything further promotion of the idea that recreational use of marijuana is completely harmless should be discouraged more aggressively.


References:

Cannabis Use is Quantitatively Associated with Nucleus Accumbens and Amygdala Abnormalities in Young Adult

Recreational Users (The Journal of Neuroscience-April 16, 2014)

Smoking cannabis could change the part of the brain dealing with motivation, according to one new study(The Independent April 16, 2014)
Cannabis use increases cancer risk study suggests (MRC News & Publications)
Marijuana May Hurt The Developing Teen Brain(Health News NPR-March 3, 2014)
Heavy Marijuana Use Alters Teenage Brain Structure(Psychology Today-March 30, 2014)

Heavy marijuana users have abnormal brain structure and poor memory (Northwestern University on Eureka Alert-December 16, 2013)

Cannabis-Related Working Memory Deficits and Associated Subcortical Morphological Differences in Healthy Individuals and Schizophrenia Subjects (Schizophrenia Bulletin- Published 12/15/13) 

Evaluation of the DNA Damaging Potential of Cannabis Cigarette Smoke by the Determination of Acetaldehyde Derived N²-Ethyl-2′-deoxyguanosine Adducts(Chemical Research in Toxicology-May 18, 2009)