A leading psychologist has warned against the assumption that smoking pot is safer than consuming alcohol.
Sir Robin Murray, a professor of Psychiatric Research at the Institute of Psychiatry at King's College London, told the Times that around a third of psychosis patients at his south London practice had been users of potent cannabis.
He said those affected were mostly young, and coming to the clinic with problems like debilitating paranoia and hallucinations.
He told the newspaper that the high cannabis-related caseload was affecting the care of other patients.
"I think we're now 100 per cent sure that cannabis is one of the causes of a schizophrenia-like psychosis," he said.
"If we could abolish the consumption of skunk we would have 30 per cent less patients [in south London] and we might make a better job of looking after the patients we have."
The warning from Sir Robin coincides with reports of a new pilot scheme in London that would offer 18- to 24-year-olds caught with a "small amount" of cannabis educational courses on the harms of drug use instead of arrest.
As more countries around the world move to decriminalise cannabis use, a recent American paper warned that the link between the drug and schizophrenia "needs our special attention".
The paper, by researchers at the California Institute of Behavioral Neurosciences & Psychology, warned that frequent users were twice as likely to suffer from schizophrenia in later life.
"Case-control studies show that earlier and higher doses of cannabis use lead to the more rapid development of psychotic symptoms," the researchers said.
"Frequent use of cannabis, especially the start of use at a younger age, doubles the risk of schizophrenia development in the future.
"Daily use of marijuana increases the risk of psychotic illness development with as much as five times higher risk in persons using high potency THC (tetrahydrocannabinol - the component of cannabis that produces the 'high')."
A separate study published in The Lancet Psychiatry in 2019 found that daily cannabis use - and especially of high potency cannabis - was "strongly linked" to the risk of developing psychosis.
Out of 11 sites across Europe and one in Brazil, the link was found to be strongest in London, where three in 10 cases were linked to high potency use, and in Amsterdam, where the rate was five in 10.
Dr Marta Di Forti, lead author from the Institute of Psychiatry, Psychology, and Neuroscience at King's College London, UK, said: "Our findings are consistent with previous studies showing that the use of cannabis with a high concentration of THC has more harmful effects on mental health than the use of weaker forms.
"They also indicate for the first time how cannabis use affects the incidence of psychotic disorder at a population level.
"As the legal status of cannabis changes in many countries and states, and as we consider the medicinal properties of some types of cannabis, it is of vital public health importance that we also consider the potential adverse effects that are associated with daily cannabis use, especially high potency varieties."
