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expert reaction to cannabis and paranoia

A study in Schizophrenia Bulletin of the effects of THC, the main psychoactive compound in cannabis, reported strong evidence that it can cause short-term paranoia and identified psychological factors that can lead to feelings of paranoia in cannabis users.


Dr Michael Bloomfield, MRC Clinical Research Fellow & Honorary Specialty Registrar in Psychiatry, Clinical Sciences Centre, London, said:

“There is now consistent evidence that heavy cannabis use, particularly during adolescence, can increase the risk of paranoid psychosis, which is typically seen in schizophrenia, a potentially devastating mental illness.  Previous research has shown that THC, when given to otherwise healthy individuals, can cause paranoia, a key symptom of schizophrenia.  This new, large and well-conducted study by a highly respected team of scientists helps explain the psychological processes underlying how THC increases paranoia.  The researchers found that THC does indeed give rise to paranoia in vulnerable people and that this happens by increasing unpleasant emotions such as anxiety.

“This new study ties in well with previous research into the mental processes that give rise to some of the symptoms of schizophrenia such as paranoia and believing in things that aren’t based in reality.  Importantly, the authors conclude that reducing unpleasant emotions could improve the paranoia associated with THC.  This further improves our understanding of how talking therapies such as CBT, can benefit the 1 in 100 people who will suffer from schizophrenia at some point in their lives.

“There is some evidence to suggest that cannabis dependence may be on the rise. Across Europe, for example, cannabis is now second only to heroin as the main illicit drug taken by patients attending specialist addictions clinics.  It’s therefore imperative for us to understand the effects of cannabis.  Our team, led by Dr Oliver Howes, has previously found that heavy regular cannabis use is associated with a lowering of the brain chemical dopamine.  Taken together, these findings tell us more about how cannabis affects the brain and the mind.  This is essential in order for us to understand how cannabis increases the risk of schizophrenia and to develop new treatments for people who have the condition and/or a problem with their cannabis use, both in terms of new medicines and better talking therapies.”


‘How cannabis causes paranoia: Using the intravenous administration of ∆9-tetrahydrocannabinol (THC) to identify key cognitive mechanisms leading to paranoia’ by Freeman et al. published in Schizophrenia Bulletin on Wednesday 16th July.


Declared interests

None declared

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