Previous research has found that people who use cannabis are at an increased risk of developing schizophrenia. However, most studies have suggested that cannabis may interact with other risk factors to cause the condition.
For the present study, the researchers examined data from 7,186,834 individuals from Danish national health registry data. All individuals were aged 16 years or older at some point between 1972 and 2016. They included equal numbers of men and women.
From analyzing the data, the researchers found that while 2% of schizophrenia cases in the country were linked to cannabis use disorder in 1995, the same figure increased to 4% in 2000, and to 8% in 2010.
The researchers say that these increases were expected given increases in the use and potency of cannabis over time. They add that the findings point towards a causal link between cannabis and the development of schizophrenia.
The study goes against previous theories stating that rates of schizophrenia remain constant over time and are independent of geographical location. This argument has previously been used to argue against a possible causal link between cannabis and schizophrenia.
"Of course, our findings will have to be replicated elsewhere before firm conclusions can be drawn," said Carsten Hjorthøj, one of the authors behind the study.
"But I do feel fairly confident that we will see similar patterns in places where problematic use of cannabis has increased, or where the potency of cannabis has increased, since many studies suggest that high-potency cannabis is probably the driver of the association with schizophrenia," he added.
The findings are "particularly important with the increasing legalization of cannabis for both medicinal and recreational uses seeming to lead to an increase in the perception of cannabis as relatively harmless and possibly in the uptake of cannabis use, especially among youth," wrote the researchers.
Sources: JAMA Psychiatry, The Lancet, Medscape, CNN