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.