Middle-aged and older adults who sought hospital or emergency room care because of cannabis use were almost twice as likely to develop dementia over the next five years, compared with similar people in the general population, a large Canadian study reported on Monday.

When compared with adults who sought care for other reasons, the risk of developing dementia was still 23 percent higher among users of cannabis, the active ingredient in marijuana, the study also found.

The study included the medical records of six million people in Ontario from 2008 to 2021. The authors accounted for health and sociodemographic differences between comparison groups, some of which play a role in cognitive decline.

The data do not reveal how much cannabis the subjects had been using, and the study does not prove that regular or heavy cannabis use plays a causal role in dementia.

But the finding does raise serious concerns that require further exploration, said Dr. Daniel T. Myran, the first author of the study, which was published in JAMA Neurology.

“Figuring out whether or not cannabis use or heavy regular chronic use causes dementia is a challenging and complicated question that you don’t answer in one study,” said Dr. Myran, an assistant professor of family medicine at University of Ottawa.