Cannabis users are more likely to commit violent crime, pioneering research has shown.
It warned those who smoke the drug regularly run an increased risk of using violence against others.
The project is the first to demonstrate that cannabis is not only linked with violent crime but is the cause.
Violent incidents monitored by the study based on the lives of more than 1,100 American psychiatric patients included assaults, attacks with weapons and rapes.
Pictured: A CCTV image of the Somali-born Muhiddin Mire, 30, (left) during a shocking rampage at Leytonstone Tube station. A court heard his addiction to skunk cannabis had altered his brain to make him believe he was being followed by MI5
Researchers said that cannabis causes violence and they found no evidence that the link is the other way round – ie that violent people are more likely to use cannabis.
There was no support, they added, for theories put forward by campaigners anxious to free the drug from the taint of links with crime.
The academics said the effect of cannabis use was clear and not diminished by other factors such as patients who were heavy drinkers of alcohol.
The study comes after a series of American states have decriminalised cannabis – despite it being stronger and more potent than the hash smoked by hippies in the Sixties – or made it available for medical use.
A number of influential figures have backed a campaign for British laws banning the drug to be relaxed, including Richard Branson, Sting and former Deputy Prime Minister Nick Clegg.
Even Prince William gave a boost to the liberalisation lobby last month when he asked a group of recovering addicts at a drugs charity about legalising banned drugs.
The Prince observed: ‘There’s obviously a lot of pressure growing on areas about legalising drugs.’
THE SHOCKING TOLL OF ATTACKS LINKED TO CANNABIS USERS
A string of serious crimes has been committed by users of skunk cannabis.
Muhiddin Mire, 30
The schizophrenic was jailed for life for the attempted murder of commuters at Leytonstone Tube station, East London, in 2016. A court heard his addiction to skunk cannabis had altered his brain to make him believe he was being followed by MI5.
Walter Pantellaro, 27
The kung fu champion was tried for kicking his way into a London flat in March and attacking a woman, 22, with a knife. She was saved by her 15-year-old brother, who was hurt as he defended her with a chair.
Pantellaro, a schizophrenic who thought he was God, told police he had taken cocaine. But tests showed the only drug in his system was cannabis.
Pictured: Walter Pantellaro, a schizophrenic who thought he was God, told police he had taken cocaine but tests showed the only drug in his system was cannabis
Nicholas Salvador, 25
A cage fighter, he was detained indefinitely at Broadmoor for beheading an elderly woman with a machete on a rampage through gardens in North London in 2015.
He was a heavy user of skunk cannabis and thought his victim was Adolf Hitler or a demon in the form of an old lady.
Matthew Graham, 29
The office worker was detained after stabbing a prostitute in the neck with a seven-inch knife in Rochdale in 2015.
He struck her with such ferocity that the handle snapped off while the blade remained lodged in her neck. The court was told he was a schizophrenic whose attack had been triggered by his use of cannabis.
Michael Adebowale, 22
The Islamist extremist was jailed for the murder of drummer Lee Rigby in Woolwich, South-east London, in 2013. His trial heard that his symptoms of psychosis were increased by heavy use of cannabis.
Frederick Russell, 28, He was tried for stabbing a homeless man near Putney Bridge Tube station, West London, in 2013. Russell was said to be a schizophrenic with a history of alcoholism and cannabis use.
Nicola Edgington, 32
She was convicted of murder after stabbing a stranger Sally Hodkin, 58, in the street with a 12-inch butcher’s knife in 2013.
Edgington had been in detention for killing her mother but had been freed. Before the stabbing, she had told a psychiatric nurse she had stopped taking her medication and had used skunk cannabis.
The latest study by five researchers from institutes based in Montreal, Canada, examined the lives of 1,136 men and women who were patients at psychiatric hospitals in Missouri, Pittsburgh and Massachusetts.
Records were gathered from interviews carried out every ten weeks for a year after their discharge.
It said patients who were using cannabis at each of these five checks were nearly two-and-a-half times more likely to have turned to violence than those who had not used the drug.
The study pointed to ‘significant findings regarding the adverse effects of cannabis use on violence’.
It found there was a ‘more constant relationship’ between cannabis and violence than between alcohol or cocaine use and violence. The researchers said the link between cannabis and violence was not two-way but ‘uni-directional’.
Contrary to claims that violent people were drawn to use cannabis, researchers found ‘it was cannabis use that predicted future violent behaviour’.
The academics said psychiatrists and medical staff should watch for cannabis users among those who had been in hospital for mental health problems. The team, led by Dr Jules R Dugre, said existing evidence on the links between cannabis and violence was ‘limited’ but their project had ‘clinical and violence risk management implications’.
Kathy Gyngell, a fellow of the Centre for Policy Studies think-tank, welcomed the ‘definitive study’ and called for official action. ‘Government has been seriously negligent,’ she said.
‘Where is the public health campaign on the risks of cannabis? If ministers had any sense they would know that we cannot afford this public health and safety crisis.
‘It must lead the Government to review their community care policy for such potentially violent individuals to better protect the public.’
The study in the journal Frontiers in Psychiatry flies in the face of claims of former drugs tsar Professor David Nutt.
He was sacked by the Labour government in 2009 for opposing the decision to reclassify cannabis from Class C to Class B. Prof Nutt has long argued that alcohol was ‘considerably more dangerous’ than the drug.
More than 20 US states have in recent years legalised cannabis for medical purposes.
Four – Colorado, Alaska, Oregon and Washington – have allowed its recreational use.
After the relaxation in 2012 in Colorado, cannabis use by students aged 12 to 18 has become the highest in the country. Figures show 57 per cent tested positive in high school tests.