The UK Misuse of Drugs Act (1971) divided controlled drugs into three groups – A, B and C – with descending criminal sanctions attached to each class. Cannabis was originally assigned to Group B, but in 2004, it was transferred to the lowest risk group, Group C.

In 2009, on the basis of increasing concerns about a link between high strength cannabis and schizophrenia, it was moved back to Group B.

Researchers from the University’s Department of Health Sciences examined admissions to psychiatric hospitals in England between 1999 and 2010 to explore whether the reclassifications in 2004 and 2009 were associated with changes in the admissions rate for cannabis psychosis.

They found that there was a significantly increasing trend in admissions from 1999 to 2004, but following the reclassification of cannabis from B to C in 2004, there was a decline until 2009. Following the second reclassification back to class B in 2009, there was once again a significant increase in admissions.

The study was led by Ian Hamilton from York’s Department of Health Sciences, an expert on the relationship between substance misuse and mental health. The results are published in the International Journal of Drug Policy.

Psychosis describes a range of symptoms, such as distortions of reality, hearing voices, difficulty in thinking and problems with motivation.

Ian Hamilton said: “Our research shows an interesting relationship between the Government’s decision to reclassify cannabis and the rate of hospital admissions for cannabis psychosis. It is significant as the Government’s argument for reclassification was made on the basis that the stronger forms of cannabis known as ‘skunk’ are more likely to lead to mental health problems such as psychosis. However, our research challenges this.

“While our study shows a statistical association between the reclassification of cannabis and hospital admissions for cannabis psychosis, it is in the opposite direction to that predicated by the presumed relationship between the two.”

The authors say the reasons for the statistical association between the reclassification of cannabis and hospital admissions for cannabis psychosis are unclear.

Ian Hamilton said: “The association is unlikely to be due to changes in cannabis use over this period, but possible explanations include changes in policing and systematic changes in mental health services unrelated to classification decisions.”