Since the nineteenth century there have been descriptions of acute schizophrenia-like psychotic symptoms in healthy people who have taken cannabis. The prevalence of cannabis use in psychotic patients being treated for the first time is approximately double that found in the general population and those patients who carry on using cannabis have earlier and more frequent relapses compared to non users. Most of the effects of cannabis are due to a substance called delta-9-tetrahydrocannabinol (THC) but there are nearly 60 other chemicals from the same family in smoked cannabis and the variations in the quality of cannabis, the composition of joints and the physiology of smokers all make it hard to assess the effect of individual substances. Researchers from the Institute of Psychiatry in London used injections to deliver a standardised dose of 2.5mg of THC (about the same quantity as there is in an average joint) to 22 healthy men to see whether this would induce psychosis. The men were sometimes given an injection of a placebo for comparison purposes. The study found that THC did induce psychotic symptoms in previously-healthy men. THC also increased anxiety and affected neuropsychological performance but there was no relationship between the participants' levels of psychosis and their levels of anxiety or neuropsychological performance.
Morrison, P.D. ... [et al] - The acute effects of synthetic intravenous delta-9-tetrahydrocannabinol on psychosis, mood and cognitive functioning Psychological Medicine 39(10), 1607-1616