Does smoking cannabis make people more violent?

The article below summarizes an academic study which claims to show that smoking marijuana causes people to beome more violent.  But I don't think the authors concerned have remotely proved that.  I myself have never smoked anything and I don't intend to so I have no dog in that fight.

I have read a great deal of the studies of the subject and I am reasonably certain that cannabis promotes psychotic delusions --    but violence is another matter.  My experience is that stoned guys are most likely to sit around passively and make profound remarks like: "Wow, man, like wow!"

So what is wrong with the study below?  Mainly the sampling -- or lack of it.  It was a study of patients released from psychiatric hospitals.  Generalizing from very ill people to normals is simply invalid, to put it mildly.

It's a common comment that any correlation between cannabis use and mental illness may simply show that mentally ill people like cannabis.  Mentally ill people are heavy users of tobacco so that is not an unreasonable view.  The patients are simply trying to self-medicate in both cases.

The authors of the current study, however, claim that they can disprove that theory and show that cannabis sends you mad, not the other way around.  But that too is an absurd claim. You can demonstrate cause only by way of a carefully controlled before and after study and that was not done on this occasion. Instead some statistical jiggery pokery was used.  But that finding too is derived from their non-sample so is again overgeneralizing.  The bottom line is that they have no data proving the matter either way.

A final concern is that the authors mainly seem to concern themselves with "skunk" cannabis, which is much stronger than other forms of cannabis and so should be studied separately.  Skunk does indeed appear to be of concern but what is true of it may not be true of all cannabis.  As toxicologists say, the toxicity is in the dose.

The academic article is Persistency of Cannabis Use Predicts Violence following Acute Psychiatric Discharge

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.

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.

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.


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