Smoking cannabis every day warps key structures of the brain, a scientific study suggests. Regular use of the drug seems to shrink the brain’s ‘grey matter’ - the cells that crunch information - according to head scans of heavy drug users. The wiring of the brain – the ‘white matter’ that connects different parts - grows to compensate for the loss of the vital cells, scientists found. But eventually that also breaks down, impairing a drug users’ ability to use and react to information. Scroll down for video . Regular cannabis use appears to shrink the brain's 'grey matter' -  the cells that crunch information - according to scientists at the universities of Texas and New Mexico . The brain scan study is one of the first to investigate the drug’s long-term neurological impact in living people. The findings add to a growing weight of evidence that suggests cannabis is more harmful than legalisation campaigners would have us believe. It comes after a review of 20 years of cannabis research, published last month by a professor at King’s College London, revealed that one in six teenagers who use cannabis become dependent on the drug, as do one in 10 adults. That review also suggested that cannabis use in teenagers doubles the risk of developing psychotic disorders including schizophrenia. The authors of the new study, from the universities of Texas and New Mexico, warn that people who take the drug in heavy quantities for prolonged periods are likely to suffer damaging effects. A definitive 20-year study into the effects of long-term cannabis use has demolished the argument that the drug is safe. Cannabis is highly addictive, causes mental health problems and opens the door to hard drugs, the study found. The paper by Professor Wayne Hall, a drugs advisor to the World Health Organisation, builds a compelling case against those who deny the devastation cannabis wreaks on the brain. Professor Hall found: . Examining brain scans, the scientists found that chronic marijuana users - who smoked an average of three times a day - had smaller average volumes of grey matter in the orbitofrontal cortex, the part of the brain involved in mental processing and decision making. The cannabis users were also seen to have more ‘white matter’ - the connections between cells which affect how the brain learns and functions. The researchers suspect that those extra connections are forged as the brain tries to compensate for the lack of crucial grey matter. But even those extra connections were seen to break up within six to eight years under prolonged cannabis abuse, they found. Dr Sina Aslan from the University of Texas said: ‘What’s unique about this work is that it combines three different magnetic resonance imaging techniques to evaluate different brain characteristics. ‘The results suggest increases in connectivity, both structural and functional that may be compensating for grey matter losses. ‘Eventually, however, the structural connectivity or “wiring” of the brain starts degrading with prolonged marijuana use.’ The team studied MRI scans of the brains of 48 adult cannabis users aged 20 to 36 who were compared with a group of 62 non-users. The orbitofrontal cortex region of the brain, where the biggest differences in cannabis users’ brains was seen, is strongly linked to empathy - the ability to sense other people’s feelings. Neuroscientists believe damage to the orbitofrontal cortex may underpin many cases of personality disorder and psychopathy. The researchers found that the effect differed markedly depending on the age they started smoking and the number of years they continued to abuse the drug. The earlier someone started smoking cannabis, the greater the structural change to the brain and the larger the growth in white matter connections. This may explain why some chronic cannabis users sometimes appear to be coping normally, effectively masking the damage to their brain, the scientists said. Examining brain scans, the scientists found that chronic marijuana users - who smoked an average of three times a day - had smaller average volumes of grey matter, pictured on an MRI scan in light blue and green, in the orbitofrontal cortex, the part of the brain involved in mental processing and decision making . But after six to eight years of continually taking cannabis the increases in structural wiring declined, they found. The scientists said that because their study represented a simple snapshot of users’ brains, and did not monitor them over time, they could not be certain that the damage was directly caused by the drug. While the statistical picture they built up suggests that the drug abuse is linked to the structure of the brain, to be absolutely sure they more research is needed, they said. Co-author Dr Francesca Filbey, also from Texas, said: ‘We have seen a steady increase in the incidence of marijuana use since 2007. ‘However, research on its long-term effects remains scarce - despite the changes in legislation surrounding marijuana and the continuing conversation surrounding this relevant public health topic. ‘While our study does not conclusively address whether any or all of the brain changes are a direct consequence of marijuana use, these effects do suggest that these changes are related to age of onset and duration of use.’ UK drugs law has been the subject of much debate in recent years. The Labour government downgraded the drug to Class C in 2004 – meaning officers did not normally arrest those caught with the drug - but reversed their decision within five years, returning it to Class B status. Celebrities including Russell Brand, Richard Branson and Sting have called for it to be decriminalised, as has Professor David Nutt, the government’s former drugs advisor.
Scientists at the universities of Texas and New Mexico found the drug does warp key structures in the brain . Regular use shrank 'grey matter' - the cells that process information . 'White matter' that connects different parts of the brain, grows to compensate but also breaks down impairing ability to react to information .