(CNN) -- The proportion of schizophrenia cases linked with problematic use of marijuana has increased over the past 25 years, according to a new study from Denmark.
In 1995, 2% of schizophrenia diagnoses in the country were associated with cannabis use disorder. In 2000, it increased to around 4%. Since 2010, that figure increased to 8%, the study found.
"I think it is highly important to use both our study and other studies to highlight and emphasize that cannabis use is not harmless," said Carsten Hjorthøj, an associate professor at the Copenhagen Research Center for Mental Health and an author of the study published in the medical journal JAMA Psychiatry, via email.
"There is, unfortunately, evidence to suggest that cannabis is increasingly seen as a somewhat harmless substance. This is unfortunate since we see links with schizophrenia, poorer cognitive function, substance use disorders, etc," Hjorthøj wrote.
Previous research has suggested that the risk of schizophrenia is heightened for people who use cannabis, and the association is particularly driven by heavy use of the drug. Many researchers hypothesize that cannabis use may be a "component cause," which interacts with other risk factors, to cause the condition.
"Of course, our findings will have to be replicated elsewhere before firm conclusions can be drawn," Hjorthøj continued. "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."
Around the world tens of millions of people use cannabis. It's legal for recreational use in 19 US states and Canada. In these and some other places, it's also approved to treat some medical conditions.
Cannabis use and cannabis use disorder has been increasing in Denmark, the study said -- a pattern that's also seen globally. Recreational weed use is illegal in Denmark but is allowed for medicinal purposes.
Cannabis use disorder is usually defined as problematic use of the drug: developing tolerance to weed; using cannabis in larger amounts or over a longer period than intended; being unable to reduce use; spending a lot of time obtaining, using or recovering from the effects of cannabis; giving up important activities and obligations in favor of cannabis; and continued use of the drug despite negative consequences.
An increase in schizophrenia?
Schizophrenia is a chronic, severe and disabling mental disorder. Its symptoms may include delusions, thought disorders and hallucinations. Worldwide, schizophrenia affects 20 million people. No cure exists, so doctors try to manage the symptoms with medications and therapy.
While one study has suggested that schizophrenia is increasing in Denmark, in other countries the picture is uncertain, said Hjorthøj. In the US, the National Institute of Mental Health said it's hard to obtain accurate estimates of the prevalence of schizophrenia because diagnosis is complex and it overlaps with other disorders.
"Many textbooks in psychiatry state that the incidence... of schizophrenia is constant over time and independent of geographical location," Hjorthøj said.
"And this has often been used as an argument against the hypothesis that cannabis could cause schizophrenia," he added. "However, it turns out that there is very little research that would support this notion."
The new study was based on data from Denmark's national health registry and included all people in Denmark born before December 31, 2000, who were 16 years or older at some point from January 1, 1972, to December 31, 2016.
The findings could help explain the "general increase in the incidence of schizophrenia that has been observed in recent years" and provides some support that the "long-observed association between cannabis and schizophrenia is likely partially causal in nature," the study said.
Legalization and regulation
The study assessed people who had a clinical diagnosis for cannabis treatment disorder, not general use of the drug, noted Terrie Moffitt, a professor and chair in Social Behaviour & Development of the Institute of Psychiatry, Psychology, and Neuroscience at King's College London.
"This study of nationwide medical records adds important evidence that patients with diagnosed cannabis use disorder are more at risk for psychosis now than they used to be," Moffitt, who studies the effects of cannabis use on the mental health of the baby boomer generation, told the Science Media Centre in London.
However, Moffitt said that most cannabis users, even those who are dependent on it, never seek treatment and many people use it recreationally without developing problems.
"It is known that people who seek treatment tend to have multiple mental health problems, not solely cannabis problems," Moffitt said. "And there are far more recreational cannabis users who manage cannabis well than cannabis-dependent users who cannot manage it."
In an editorial that accompanied the study, Tyler J. VanderWeele, a professor in the Departments of Epidemiology and Biostatistics at the Harvard T. H. Chan School of Public Health, said that estimates in the study could be conservative because of underdiagnosis of cannabis use disorder.
"Cannabis use disorder is not responsible for most schizophrenia cases, but it is responsible for a nonnegligible and increasing proportion. This should be considered in discussions regarding legalization and regulation of the use of cannabis," VanderWeele wrote in the commentary.
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