Legalising cannabis would result in soaring numbers of people suffering from schizophrenia-like psychosis, one of Britain’s top psychiatrists has warned. 

Evidence now shows that when the drug is legalised, greater numbers smoke it more frequently and in stronger varieties. 

These factors increase the incidence of cannabis-related psychosis, according to Professor Sir Robin Murray, an authority on the risks of the drug to mental health. 

He predicted that ‘big cannabis’ firms with scant regard for people’s health will ‘seduce’ the Government into reforming the law – and the State will then find itself in thrall to the new industry because of the tax and jobs it provides.

Hand holding a marijuana leaf on a background of blue sky – stock image

The warning is a major intervention from Professor Murray, who told The Mail on Sunday growing evidence about the harm cannabis causes had made him change his mind about legalising the Class B drug. 

Previously the expert – a professor at the Institute of Psychiatry at King’s College London – had supported tightly controlled legalisation married with public education campaigns spelling out the risks.

But he said the experience of places that had decriminalised or legalised it – from Portugal and the Netherlands to swathes of North America – had made him think again.

Besides clear evidence that prolonged use greatly raises the risk of serious mental illness, he said he was also forced to rethink his position by the explosive growth of a ‘wild west’ cannabis industry.

Marijuana joints next to a jar filled with loose marijuana waiting to be rolled

He said: ‘I didn’t appreciate how big the cannabis industry was going to be. These guys in Canada and California, they are setting out that the cannabis industry will be as big as the tobacco industry. And of course they can’t be trusted.’

Professor Murray outlined his concerns in the journal JAMA Psychiatry with Australian public health expert Wayne Hall.

‘In those US states that have legalised cannabis, the price has fallen and both cannabis use and dependency has increased among adults,’ they wrote.

One US study they cited found 30 per cent of users were now dependent, triple the proportion in the 1990s, with dependency growing as the strength of the drug increased. That finding accords with research published last week which found the drug can be highly addictive and can cause serious withdrawal symptoms.

Some reports found cannabis use increases the risk of depression and suicide, said the pair, ‘but by far the strongest evidence concerns psychosis’.

‘Numerous prospective studies have shown cannabis use carries an increased risk of later schizophrenia-like psychosis,’ they warned. 

In fact, ten of 13 such studies showed users had ‘a significantly increased risk of psychosis’ while ‘two of the remaining three showed a trend in that direction’. Those who smoked ‘high-potency cannabis’ daily saw their risk of psychosis increase ‘up to nine-fold’.

Last night Professor Murray denied being alarmist. ‘It took 40 years before it was accepted that smoking caused lung cancer,’ he said.

His warnings come amid growing calls to legalise cannabis. At the last Election the Liberal Democrats proposed legalising cannabis while

Labour said it would ‘progress clinically appropriate prescription of medical cannabis’.

The Government insists it has no plans to legalise the drug.