Post-traumatic stress disorder (PTSD) may significantly increase the risk of dementia later in life, according to a new study.

The researchers found that people with a history of PTSD were up to two times more likely to develop dementia than those who never had PTSD.

"Our study provides important new evidence of how traumatic experiences can impact brain health, and how the long-term effects of trauma may impact the brain in many ways increasing vulnerability to cognitive decline and dementia," said senior study author Dr. Vasiliki Orgeta. She's an associate professor of psychiatry at University College London.

"PTSD, which appears to be common among people who have been hospitalized with COVID-19, remains an underdiagnosed, undertreated, and under researched mental health condition, yet it can have serious long-term consequences," Orgeta said in a university news release.

For the study, researchers analyzed 13 studies from four continents that included a total of nearly 1.7 million people. Data from eight of the studies showed that people with PTSD had a 61% higher risk of dementia up to 17 years later.

When the investigators examined data from two studies that used different methods, they found that PTSD was associated with a twofold higher risk of dementia.

The study also found that dementia risk among people who'd had PTSD was more than two times higher in the general population than among veterans.

This may be because veterans are typically more likely to receive treatment for PTSD than people in the general population (in the countries included in the analysis), so the findings suggest that treating PTSD may reduce dementia risk, the study authors noted.

"A lot of people with PTSD don't access treatment, sometimes due to a lack of mental healthcare capacity but also because of stigma, which often keeps people away from seeking help. We now have more evidence of how traumatic experiences and accessing treatment could have a long-lasting impact for individuals and influence future risk of developing dementia," Orgeta said.

The risk of dementia among people with PTSD may be even higher than these findings suggest because PTSD also increases the likelihood of other dementia risk factors, such as depression, social isolation and heavy drinking, according to the researchers.

It's not clear how PTSD increases dementia risk, but perhaps hypervigilance and recurrent re-experiencing of trauma contribute to stress-related activity in the brain, the team added.

The study was published Sept. 16 in the British Journal of Psychiatry.