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Cannabis isn't a cause of psychotic conditions like schizophrenia in the sense that it directly leads to psychosis. That's obvious - we all know people who've smoked for years and haven't got schizophrenia; we also know people who have psychotic symptoms who haven't used any drugs..

But this is a hot potato of a question because it depends on what's meant by "cause". There are no specific causes as no single cause has been identified -
even genes only carry a 50% risk. it's better to think of "risk factors" - factors which increase the risk of it developing. Some people put birth complications as the main causal factor in about 40% of cases of schizophrenia.

But that doesn't mean that cannabis definitely has no causal role. No single cause for psychosis has ever been identified - it's almost always a bit of a melting pot of risk factors like genes, birth complications, racism, living in cities, trauma, the list is almost endless. The thing is that if more than one risk factor is present, the chances of developing a psychotic condition goes up exponentially, or as researchers put it, the risk factors interact.

Cannabis use might be one of these risk factors, especially for children or young teenagers and especially if they use a lot of it - the more smoked, and the younger the user - the bigger the risk.

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Tetrahydrocannabinol has cardiovascular effects, and sudden deaths have been attributed to smoking cannabis.10 Myocardial infarction is 4.2 times more likely to occur within an hour of smoking cannabis.11 However, despite these alarming facts, there is no evidence at present on whether smoking cannabis contributes to the progression of coronary artery disease, as smoking cigarettes does. More studies of the cardiovascular and pulmonary effects of cannabis are essential.

It may be argued that the extrapolation from small numbers of individual studies to potential large scale effects amounts to scaremongering.

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Intoxication with cannabis leads to a slight impairment of psychomotor and cognitive function, which is important for those driving a vehicle, flying an aircraft or operating machinery (DH Q 197). The Department of Health rate this as "the major concern from a public health perspective" raised by recreational use (p 46), and Professor Hall considers it the most serious possible short-term consequence of cannabis use, both for the user and for the public (p 222).

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