MORE people are being taken to hospital with “cannabis psychosis” since the drug was given a higher-risk rating four years ago, York researchers have said.

Cannabis was moved to the lowest drug-risk group – class C – in 2004, but was reclassified as class B in 2009 following concerns about a link between the highest-strength substance, known as “skunk”, and schizophrenia.

Teams from the University of York’s department of health sciences looked at admissions to English psychiatric hospitals between 1999 and 2010 to see whether the two reclassifications were connected to changes in levels of treatment for cannabis psychosis. They found the number dropped while cannabis was a class-C drug, but had shown a “significant” increase since it returned to class B, rather than the decrease it was intended to provide.

Ian Hamilton, of the department, an expert on the relationship between substance misuse and mental health, said: “The Government’s argument for reclassification was made on the basis that the stronger forms of cannabis are more likely to lead to mental health problems such as psychosis, but our research challenges this.”

He said possible explanations included changes in policing and mental health services which were not connected to classification decisions.