In a historical context, cannabis and marijuana have been used for thousands of years. The earliest mention of the substance in written record is from China in roughly 2,700 BC, and since then it has occured in increasing quantities across the globe. In 1850, it was even mentioned in the early-1800s edition of the United States Pharmacopeia. However, it wasn’t until the 1850s that the rise of recreational marijuana began to rear its head.

A new study by the Lancet Group—a weekly medical journal, founded in 1823, who specialise in topics relating to psychiatry and other areas of healthcare—looked at the way in which cannabis use (and abuse) contributes to the incidence rate of psychosis and psychotic behaviour across the European nations. The study assessed individuals aged between 18 and 64 years of age who had presented as displaying psychotic behaviour, and further individuals chosen at random from local populations who showed normal, rational behaviour. In total the study assessed over 900 first-stage psychosis patients and compared them to 1,200 control patients. The results concluded that daily use of cannabis did, indeed, increase the risk of a patient developing psychotic behavioural patterns.

Apart from this study, Qz concluded that as many as 30% of first-episode psychosis incidents in London, and a whopping 50% of incidents in the city of Amsterdam, could have been prevented if particularly strong cannabis was less easily accessible in said cities.

The conclusions are not limited to this study, either. Earlier studies came to the same conclusion and in 2016, scientists made official warnings about the dangers of consuming cannabis, especially high-potency variants. Furthermore, a paper published in 2013 pointed out the link between cannabis/marijuana and overly violent behaviour in Washington DC; shortly after legalizing the drug, the rates of violent behaviour spiked.

Furthermore, according to research which compared a number of unrelated studies into the effects of cannabis consumption, which was posted in the U.K. Newspaper, the Daily Mail, and shared by the NHS, even a single smoke of cannabis has the potential to increase the incidence rate of schizophrenia in an individual by 40% and that cannabis “could be to blame for one in seven cases of schizophrenia and other life-shattering mental illness” issues. The nature of the pooling of numerous sources of information make this a fairly reputable and reliable resource.

All in all, it is—unsurprisingly—obvious that a high consumption of cannabis and marijuana can lead to a multitude of mental and behavioural disorders, however the effects of small scale cannabis intake is still hard to determine. For one, it cannot be said for certain whether it is the marijuana itself that causes psychosis, or if instead those who are genetically predisposed to psychosis are also more inclined to seek it out to begin with. Adding to this, research into marijuana is also highly regulated and as such studies are hard to come by, and more information would help to validate or disprove the above claims.