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People who regularly smoke cannabis ‘are nearly three times more likely to be violent’, study finds

  • A study of 300,000 teens and young adults found cannabis use alters the brain 
  • Addicts may also suffer withdrawal symptoms that could cause anger problems 
  • Experts say the link between cannabis use and violence is a ‘neglected area’

People who regularly smoke cannabis are almost three times more likely to commit a violent offence as those who abstain from the drug, new research has found.

Scientists involved in a landmark study of almost 300,000 teenagers and young adults believe that over time, prolonged cannabis use profoundly alters the brain, making the user less able to control their temper.

In addition, the research found addicts may also suffer from withdrawal symptoms, making them irritable and prone to lashing out.

Psychiatrist Professor Sir Robin Murray, a world-leading expert on the neurological impact of the drug, said the link between cannabis use and violence was a ‘neglected area’. 

Student Femi Nandap (above) stabbed public health expert Jeroen Ensink to death outside his home in North London in December 2015. Forensic psychiatrist Dr Samrat Sengupta, of Broadmoor Hospital, told the Old Bailey that the student’s heavy cannabis habit had triggered a genetic psychotic illness

Student Femi Nandap (above) stabbed public health expert Jeroen Ensink to death outside his home in North London in December 2015. Forensic psychiatrist Dr Samrat Sengupta, of Broadmoor Hospital, told the Old Bailey that the student’s heavy cannabis habit had triggered a genetic psychotic illness

Commenting on the study’s findings, he said: ‘This is not a surprise for those of us who follow the scientific literature or see patients who heavily use cannabis.

‘However, it may be a surprise to the many who think cannabis is a chill-out, anti-violence drug.’

Britain has been plagued by a succession of brutal killings linked to cannabis in recent years. 

In some of the cases, lawyers have argued the perpetrators should not be found guilty of murder because they were suffering from psychosis, a mental condition now understood to be exacerbated by smoking strong cannabis.

Among the killers was student Femi Nandap, who, in December 2015, stabbed public health expert Jeroen Ensink to death outside his home in North London. 

Mr Ensink, 41, had popped out to post cards announcing that his wife Nadja had given birth to their daughter.

Forensic psychiatrist Dr Samrat Sengupta, of Broadmoor Hospital, told the Old Bailey that the student’s heavy cannabis habit had triggered a genetic psychotic illness. 

Mr Ensink, 41, had popped out to post cards announcing that his wife Nadja had given birth to their daughter when he was stabbed to death by mentally ill Femi Nandap

Mr Ensink, 41, had popped out to post cards announcing that his wife Nadja had given birth to their daughter when he was stabbed to death by mentally ill Femi Nandap 

Nandap, then 23, was given an indefinite hospital order after admitting manslaughter on the grounds of diminished responsibility.

The researchers decided to examine 30 individual studies examining the link between cannabis use and violence because ‘the [scientific] literature has shown that cannabis use may lead to violent behaviours and aggression; however, this association has been inconsistent’ – with some studies showing a relationship and others not.

The team from Montreal University in Canada discovered 26 of the 30 studies showed a tendency towards higher levels of violence among cannabis users. 

When they pooled the results – meaning they were looking at a combined group of 296,815 teens and adults under 30 – they found users were more than twice as likely (2.15 times) to have committed a violent offence as non-users. 

Among ‘persistent heavy users’, the risk of violence was 2.81 times higher.

The study found that prolonged cannabis use profoundly alters the brain, making the user less able to control their temper (file photo)

The study found that prolonged cannabis use profoundly alters the brain, making the user less able to control their temper (file photo) 

Writing in the American Journal Of Psychiatry, they said: ‘This study suggests that cannabis use appears to be a contributing factor in the perpetration of violence.’

Even when accounting for different life circumstances which might mean cannabis users were more likely to grow up in violent surroundings, they concluded that ‘the effect remained significant’.

Citing neurological research, they said cannabis use during adolescence ‘may cause deterioration of neural structures associated with inhibition and sensation-seeking’, adding: ‘Such neural deficits are expected to limit one’s ability to suppress the urge to act out violently and heighten the risk of developing antisocial behaviours in adulthood.’

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