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There are increasing concerns about the harmful effects of cannabis use, particularly in terms of psychosis, but also dependence and possible cognitive dysfunction.  
Although most cannabis users experience at least transient cognitive impairments, only a minority ever develop psychosis or become dependent on the drug. So what determines an individual’s vulnerability to experiencing these harmful effects of cannabis?  
Cannabis contains a myriad of different chemicals, about 70 of which are unique to the plant and called cannabinoids. The main psychoactive ingredient is D9-tetrahydrocannabinol (THC) and this produces the effects that users seek2.  When given intravenously to healthy humans, THC produces psychotic-like and anxiogenic effects3;4.  In contrast, cannabidiol (CBD), another major constituent of most strains of cannabis, appears to have anti-psychotic properties5, is anxiolytic6 and may be neuroprotective in humans7. The relative THC/CBD ratio of cannabis varies greatly. Levels of CBD can range from virtually none to up to 40%. Higher levels of THC are found in hydroponically grown varieties like ‘skunk’ and in genetically modified strains which are increasingly common in the UK.
In this talk I will outline what is already known about vulnerability factors before discussing our own recent and ongoing research on the influence of THC and CBD.  It is emerging from this work that CBD can be protective against some harms and may have important therapeutic uses in certain psychological disorders.