INDIVIDUALS who suffer concussions are at an increased risk of up to four times for suicide even years after their injury, a new study from Canada suggests.

Researchers examined the medical records of 235,110 patients with concussions that weren’t serious enough to require hospitalization throughout a 20-year period in Ontario. Throughout an average follow-up period of nine years, they found that 667 had committed suicide. This is equivalent to 31 suicides per 100,000 each year, over three times higher than the suicide rate in the general population, according to the results.

The study further found that those who suffer concussions on weekdays are at a three times greater risk for suicide, while weekend incidents result in a four times greater risk.

“We know that a concussion can cause lasting changes in the brain that can alter mood, perhaps resulting in [behavior] changes including impulsivity,” Donald Redelmeier, a researcher and physician at Sunnybrook Research Institute, said in a press release.

Researchers said they don’t know the reason for the elevated risk of weekend concussions. However, they posited that it could be due to a number of possibilities, among which include that those who are injured on Saturdays and Sundays may suffer during recreational activities and may receive care later compared to those injured on the job on weekdays.

“It’s possible that we’re seeing greater suicide risk linked to weekend concussions due to risk-taking associated with recreation or misadventure, whereas weekday injuries may be linked to employment hazards. We may also be seeing an effect of self-blame if the injury event was self-initiated,” Redelmeier said in the release.

Study authors further noted that emergency rooms are less likely to be adequately staffed on weekends, thus resulting in less adequate care and treatment that could have a long-term effect on a concussion patient’s health.

On average, suicides occur about six years after a concussion, researchers noted.

Each year, up to 3.8 million concussions occur, according to the US Centers for Disease Control and Prevention.

Previous studies have examined the effect of concussions on athletes, which have revealed that those such as football players who have experienced four concussions were more likely than individuals without the injury to demonstrate depressive symptoms, TIME reported.

This particular study focused on adults in the general population. It was published Monday, Feb. 8, in the Canadian Medical Association Journal.