A powerful “skunk-like” cannabis is associated with one in four new cases of psychosis, a study has found.

Scientists at King’s College London also found the potent form of the Class B drug led to a three-times higher risk of psychosis in casual users, rising to a five-fold risk in those who used it every day.

The findings of the six-year study, published in the Lancet Psychiatry journal, suggest strong forms of cannabis can lead to serious mental illness, the researchers said, as they called for greater public awareness.

Sir Robin Murray, Professor of psychiatric research at the Institute of Psychiatry, Psychology and Neuroscience IoPPN at King’s and senior researcher on the study, said: “It is now well known that use of cannabis increases the risk of psychosis. However, sceptics still claim that this is not an important cause of schizophrenia-like psychosis.

“This paper suggests that we could prevent almost one quarter of cases of psychosis if no-one smoked high potency cannabis. This could save young patients a lot of suffering and the NHS a lot of money.”

While skunk was associated with psychotic episodes, the milder form of cannabis known as hash had no such link, the scientists said.

Dr Marta Di Forti, lead author on the research, said: “The results show that psychosis risk in cannabis users depends on both the frequency of use and cannabis potency. The use of hash was not associated with increased risk of psychosis.

“As with smoking tobacco and drinking alcohol you need a clear public message.

“When a GP or psychiatrist asks if a patient uses cannabis it’s not helpful; it’s like asking whether someone drinks. As with alcohol, the relevant questions are how often and what type of cannabis. This gives more information about whether the user is at risk of mental health problems; awareness needs to increase for this to happen.”

The study looked at 800 people aged between 18 and 65 in south London, including 410 who had suffered psychosis and 370 healthy patients.

The researchers highlighted south London had one of the highest recorded rates of psychosis patients and samples of skunk seized in the area had high levels of the chemical tetrahydrocannabinol (THC), linked to psychotic episodes such as hallucinations.

The report said: “In view of the high prevalence of skunk use in our study population, if a causal role for cannabis is assumed, skunk use alone was responsible for 24 per cent of those adults presenting with first-episode psychosis to the psychiatric services in south London.

“Our findings show the importance of raising public awareness of the risk associated with use of high-potency cannabis, especially when such varieties of cannabis are becoming more available.

“The worldwide trend of liberalisation of the legal constraints on the use of cannabis further emphasises the urgent need to develop public education to inform young people about the risks of high-potency cannabis.”

PA