Cannabis causes mental illness and even one-off users show signs of behaviour linked to schizophrenia, pioneering research showed yesterday.

It found in tests on 22 healthy students and academics that half showed an ‘acute psychotic reaction’ when injected with the main active ingredient of the drug.

And it detected a series of similarities to symptoms of schizophrenia in their responses.

The research, the first to try to chart exactly the impact of doses of cannabis on mental functions and mood, is one of the clearest indicators yet that the drug is a primary cause of mental illness.

It suggests that current Home Office advice to young people, which says that the drug is dangerous only to those who already have mental health problems, is misinformed and misleading.

Despite an overall decline in UK cannabis use over the past eight years, the number of under-25s to have tried the drug rose last year to nearly one in five.

The research team from the Institute of Psychiatry at King’s College, London, injected the active ingredient of cannabis, THC, into 22 healthy university students and staff, observed their behaviour, and asked them to take tests and answer questionnaires.

Their report, published in the Psychological Medicine journal, said: ‘The findings confirm that THC can induce a transient, acute psychotic reaction in psychiatrically well individuals.’

The sentiments most commonly felt by the subjects included ‘people seem to be dropping hints about you or saying things with a double meaning’ and ‘you hear your own thoughts being echoed back to you’.

They noted that after the THC injections, the subjects showed ‘marked deficits in working memory and executive functioning and a trend towards impaired episodic memory’. All three are associated with schizophrenia.

As the results were seized on by campaigners for tougher drug laws, a Home Office spokesman said: ‘We have always said that cannabis is a harmful drug.’

Daily Mail