One of the more compelling arguments against cannabis use is its link to schizophrenia and other symptoms of psychosis. The actual risk is hard to pin down, because, surprisingly, you’re not allowed to give thousands of people a drug just to see how many develop schizophrenia. But according to one study in the UK , over 20,000 users would have to abstain to prevent one case of schizophrenia.
Although the odds appear to be long, one limitation of this estimate is that it’s based in part on studies looking at weaker strains of cannabis than are seen today. Cannabis contains a range of chemical compounds called cannabinoids, and the amount of each one in any given plant can vary dramatically. The one that recreational users care most about is called Δ9-tetrahydrocannabinol, or usually just called “THC”. This is the one that gets people high, but it’s also the part of the plant linked to psychosis.
Can legal CBD oil help?
Another cannabinoid that many users don’t know about – but really should – is cannabidiol, or CBD. This has no effect on your mental state, but it may actually protect against psychosis. Years ago the average joint would contain about 4% each of THC and CBD. Nowadays stronger strains have been developed, containing up to 14% THC and almost no CBD at all. Research suggests that smoking these strains, without the protective effect of CBD, may increase the risk of psychosis.
Research conducted in Britain found a novel way of testing the impact of CBD on psychosis symptoms — hair sample analysis . Traces of both THC and CBD end up in the hair, so this can help researchers determine not only if someone has been smoking cannabis, but also the ratio of THC to CBD that they consumed. They split the people into three groups:
• THC Only – suggesting the use of stronger strains with no CBD
• THC+CBD – suggesting the use of strains with higher CBD levels
• No Cannabis – suggesting no use of cannabis
Then they gave participants a standardized questionnaire used by psychologists to identify symptoms of psychosis. The scores of the people in the THC+CBD group were the same as the scores as the people in the no cannabis group. In other words, people who smoked cannabis with higher CBD levels had the same amount of psychosis as people who don’t smoke at all. However, people in the THC Only group – the ones smoking the stronger strains without the CBD in them – scored twice as high on this test, suggesting they experience symptoms of psychosis a lot more often.
CBD has no negative side effects
This research is not conclusive, but it is backed up by several other studies. For example, one experiment conducted in Germany compared CBD with amisulpride, one of the most effective treatments for symptoms of schizophrenia, to determine their impacts on schizophrenic symptoms. The results showed that CBD produced the same improvements in symptoms as amisulpride. However, amisulpride triggered numerous side-effects, including weight gain, movement disturbances, and sexual dysfunction, while CBD produced no side effects. 
Further research is being conducted to bolster the evidence base, but so far the data suggests that:
• Cannabis users consuming strains with lower THC and higher CBD levels (or who take additional CBD through other sources) are at greatly reduced risk of experiencing psychosis.
• People who already experience psychosis can find some improvement by using CBD, without the usual side-effects of antipsychotic medication.