Cannabis is not a cause of schizophrenia, says biggest study to date

The link between substance abuse and mental health problems has always been a hotly debated scientific topic, both by scholars and society. In fact, a quick Google search will produce an endless list of articles with headlines reporting that evidence has been found supporting the existence or otherwise of this or that cause-and-effect relationship. A good example of this is schizophrenia, which has long been associated with cannabis use.

2018, however, has seen the publication of various studies suggesting that cannabis use is by no means a trigger of schizophrenia. Among them, the biggest study to date, which involved a sample of over 180,000 people, adding to the credibility of the findings.

Rise in cannabis use and cases of schizophrenia is not proportional

For the second year running, the King's College London has published the International Cannabis Survey. This year, 1,231 cannabis users reported on their use experience, including use-related pleasure and any possible psychotic effects.

The answers suggested that those who reported pleasant experiences were more likely to keep using the drug, while those who described the experience in negative terms were more likely to give up use. Hence, people reporting negative experiences, which arguably includes schizophrenics, are more likely to phase out use, says the study.

The researchers also said that there are many more heavy cannabis users than people suffering from schizophrenia - the disease affects about one per cent of the population - yet another argument against the belief that cannabis may trigger the disease. Also, the rise in cannabis use is not proportional to new schizophrenia cases.

While the King's College London's study may represent an interesting, evidence-supported new approach to assessing the link between cannabis and schizophrenia, the size of the survey sample was not big enough for the otherwise interesting data to be considered conclusive.

A sample of over 180,000 people

Shortly after the release of the King's College London's survey, the International Cannabis Consortium published another international study, conducted by researchers from the Vall d'Hebron Barcelona Hospital Campus, among others. Similarly to the British survey, the results suggested that there is not enough evidence for a causal link from cannabis use to schizophrenia.

With a sample of 184,765 patients, this is the biggest study yet on the link between cannabis and mental health problems, quintupling the number of respondents of the second biggest study.

The results are quite surprising when compared to the traditional approach from previous studies. The researchers explained that the human gene code may differ slightly from people to people, leading to slight, random differences such as the so-called single-nucleotide polymorphism (SNP), i.e. a variation in the building blocks of DNA and RNA, known as nucleotides.

The study reveals genetic overlap

Mendelian randomization analyses revealed 35 genes associated with cannabis use. Differences is some of these genes may account for a higher tendency to smoke pot. But most surprising of all, up to 16 of these genes were found to be associated also with mental health problems, among which schizophrenia. This phenomenon is known as genetic overlap and was used by the researchers to prove that, while there is a link between cannabis use and schizophrenia, there is weak evidence for that link to be causal.

Contrary to what was believed to be the most likely explanation, the researchers think it is more likely that schizophrenia leads to cannabis use, rather than the opposite. In fact, the study suggests that schizophrenics are genetically predisposed to feel drawn to cannabis, and are therefore at a higher risk of developing a habit - which might have contributed to the belief that cannabis is a cause of schizophrenia.

Another finding of the study was that cannabis is more likely to be taken by schizophrenics trying to self-medicate than to cause the disorder, once again adding to the belief that cannabis is a trigger of the disease.

While the results from these two studies cannot be taken as conclusive, they are a highly positive step forward that may prove essential to advance research on the true nature of the link between schizophrenia and the use of substances such as cannabis and alcohol, beyond stereotypes and clichés. In any case, the use of verified evidence remains key, as only thus will it be possible to effectively address diseases like schizophrenia.