This has interesting implications for the claims of cannabis leading to mental illness as promoted by Prof Robyn Murray et al. In the UK, cannabis is usually smoked with large amounts of tobacco in joints, a fact that seems to have been ignored in most if not all of the studies into cannabis and mental health. As a result most cannabis users in the UK are very heavy tobacco smokers.

Despite lobbying over many years, the UK government has consistently refused to run a safer use campaign aimed at cannabis users to deter the use of tobacco in joints.  What makes this worse is these claims of "cannabis psychosis" have been used by the UK government to oppose cannabis law reform.