A new book from US journalist Alex Berenson claims that increasing cannabis use in America is causing a ‘black tide of psychosis’ and ‘red tide of violence’.

In the publication; Tell Your Children: The Truth About Marijuana, Mental Illness and Violence he goes on to say that a ‘rise in cannabis induced violence will worsen’ as global consumption grows.

Closer to home, claims have been made in the Sunday Times newspaper that cannabis use is linked to some of Scotland’s most notorious teenage killers in the wake of the conviction of a 16-year-old boy for the murder of Alesha MacPhail on the island of Bute, last year.

Aaron Campbell told police he was ‘really stoned’ after being found guilty of creeping into the six-year-old’s family home, taking her from her bed, raping and suffocating Alesha and leaving her with 117 injuries.

In February this year the paper highlighted a further two teenage Scottish murderers in which cannabis use was said to play a role, as well as similar crimes elsewhere in the UK. However two prominent US scientists have described these claims as ‘misinformed and reckless’. Carl Hart is the chairman and Ziff professor of psychology and psychiatry at Columbia University and Charles Ksir is professor emeritus of psychology and Neuroscience at the University of Wyoming.

Writing in The Guardian they say the evidence from their research is that ‘aggression and violence are highly unlikely outcomes of marijuana use’.

“Based on our own laboratory research, during which we have given thousands of doses of marijuana to people – carefully studying their brain, behavioural, cognitive and social responses – we have never seen a research participant become violent or aggressive while under the influence of the drug, as Berenson alleges.”

They also compare Mr Berenson’s book to a re-run of the US ‘Reefer Madness’ rhetoric in the 1930s. This narrative prompted by Drugs Czar Harry Anslinger – that cannabis use prompts violence – was partially responsible for the global outlawing of a drug which have previously been viewed as beneficial, and, at worst benign.

Consultant Psychiatrist Prof Adam Winstock, the founder of the annual Global Drug Survey – in which 60,000 cannabis users detail their habits and use – does have concerns on the impact of early, onset cannabis usage.

He says: “As someone who has worked with people who use cannabis for over 20 years my experience is that the relationship between cannabis and psychosis is limited to vulnerable groups, and those people who use it early in life.

“The idea that cannabis use is not generally helpful in young people should not really be challenged. Most people agree that this is the time you have optimal brain and emotional development, and when you’re developing it’s best not to be chucking loads of chemicals in; whether it’s alcohol, or weed, or anything else.”

Prof Winstock, of University College London, says today’s street cannabis is much more potent than it has been in the past.

Most research shows that cannabis with a high THC (Tetrahydrocannabinol – the cannabinoid which gets people ‘high’) content tends to have more psychoactive side-effects. Commonly coming with a 15% or greater THC content ‘recreational’ cannabis resin is over three times stronger than the strains available 20 years ago.

Prof Winstock says: “This is now an issue because it so difficult to get anything else. For people, who don’t want a high potency strain it’s now almost impossible.”

Canada was one of the first countries to legalise medical cannabis usage and initially it was was not prescribed to anyone under 25, and while most states now have an age limit of 18, or 19 there is talk of raising the limit to 21 in some jurisdictions.

But Prof Winstock is sceptical of the widely held view that cannabis causes depression: “Early onset cannabis use is more a marker of things going wrong in your life and those people have an elevated risk of depression. Heavy use leads to complications in home lives and relationships.”

As a supporter of the recent changes to UK legislation Prof Winstock added: “I don’t think the medical use of cannabis will complicate this issue. A regulated market will be able to address potency and change issues, and delay the onset of early cannabis use.”

The All Party Parliamentary Group (APPG) for Drug Policy Reform’s official report, concludes that most literature supports a causal hypothesis between cannabis use and psychosis, particularly if usage starts at an early age and if the individual has a genetic predisposition to psychosis.

But the report says ‘it is unlikely that any one environmental factor – such as cannabis use – or any one gene can cause schizophrenia on its own’.The author of this report, neurologist Prof Mike Barnes, dismissed links between cannabis use and violence as ‘deluded wishful thinking’ by its ‘ill-informed opponents’.

He says: “The majority of good quality studies do confirm that there is a link between cannabis use and onset of schizophrenia or psychotic symptoms. The overall evidence suggests that cannabis can make symptoms worse in individuals already suffering from psychosis. “Also, it is likely that cannabis use in young people reduces the age of onset of psychosis.

“The prognosis of schizophrenia is probably worse in individuals who have used cannabis. “People who have a genetic predisposition to developing psychosis (often a family history) are more likely to develop psychotic symptoms following cannabis use.

“Thus it would be wise to suggest that those who have had, or are having, a schizophrenic illness or psychosis or a family history of schizophrenia or psychosis should avoid cannabis. This is particularly true in young people.”