(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 have 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 a 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 negat