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© Reuters / Ben Nelms
Chronic and heavy marijuana use causes the brain's gray matter literally to become smaller. However, the white matter compensates for the damage by increased neuron connectivity, US researchers recently discovered.

The team of scientists claims it is the first to comprehensively describe the puzzling changes in human brain's structure and function, associated with the cannabis use, which "remains equivocal."

Overall the IQ suffers along with the orbitofrontal cortex (OFC), a part of the brain whose volume decreases - and medics notice that its damage often leads to disinhibited behavior. That is, poor social interaction, excessive swearing and hypersexuality.

To obtain such abnormalities on a detectable level, as participants demonstrated on average, one has to consume the drug about three times per day. But the term 'chronic user' is applied to usage of at least four times per week.

Another effect of a long-term marijuana use revealed in the tests is a boost of connectivity in white matter, which is responsible for transmission of signals, thus affecting how the brain functions. However, this process gradually slows down after six to eight years of continual use.

That it is the reason behind the fact that such frequent users "seem to be doing just fine," Francesca Filbey, associate professor at the University of Texas at Dallas, said.

According to the study, gray matter could be much more vulnerable than white matter to the effects of delta-9-tetrahydrocannabinol (THC). This is the main psychoactive ingredient in the plant of cannabis, which binds to cannabinoid receptors, abundant in the brain.
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© Image from the study published in PNAS on Nov. 10
Group comparison of the gray matter volume demonstrates significant reduction in bilateral orbitofrontal gyri in marijuana users
Now, the neuroscientists are set to determine whether the brain changes revealed are subject to the frequency of marijuana use, to giving it up completely, and if they could be a direct result or just a predisposing factor.

Their study, 'Long-term effects of marijuana use on the brain', was published Monday in Proceedings of the National Academy of Sciences (PNAS) journal.

The research team got multiple magnetic resonance images (MRIs) of 48 adult marijuana users and 62 non-users aged 20-36. The researchers also controlled gender, age and ethnicity, as well as tobacco and alcohol use.

"What's unique about this work is that it combines three different MRI techniques to evaluate different brain characteristics," Sina Aslan, another author of the study, revealed.

"The results suggest increases in connectivity, both structural and functional that may be compensating for gray matter losses. Eventually, however, the structural connectivity or 'wiring' of the brain starts degrading with prolonged marijuana use," she added.

No relationship between moderate cannabis use and IQ for teenagers - UK study

Recent and extensive research carried out in the UK, has shown no connection between occasional adolescent marijuana use and teenagers' poorer educational performance.

The long-term study, which has been following the health of children born in the early 1990s, argues that other previous findings - suggesting lower IQ in cannabis users - may have resulted from social habits and lifestyle typically associated with pot abuse, rather than marijuana itself.

"It's hard to know what causes what - do kids do badly at school because they are smoking weed, or do they smoke weed because they're doing badly? This study suggests it is not as simple as saying cannabis is the problem," said Claire Mokrysz of University College London.

However, the study warned that heavier cannabis use by teenagers may lead to "slightly poorer" exam performance.

Another study, also carried out in Britain, has shown the bright side of the matter, saying cannabis could be used to prevent cancer spreading. According to scientists, marijuana's main psychoactive ingredient - THC, could be useful for shrinking tumors.