A single TBI characterised as "severe" increased the risk by 35 per cent while a single "mild" TBI or concussion increased the risk by 17 per cent, revealed the research.
Using the health records of 15,000 patients, the study found assessment of pupil reactivity - named GCS-Pupil or GCS-P - would have improved doctors' ability to predict patients' conditions six months after a brain injury.
"Individuals with a history of traumatic brain injury, including those with less severe injuries have an increased risk of developing dementia, even decades after the injury", said study leader Dr. Jesse Fann.
The study is based on analyzing 36 years of medical records for almost 2.8 million people in Denmark over age 50, looking at their histories of brain injuries, dementia and other medical conditions.
Researchers found the "fully adjusted risk of all-cause dementia in people with a history of TBI was higher than in those without a history of TBI, as was the specific risk of Alzheimer's disease".
The researchers identified a cumulative effect, with dementia risk rising with repeated episodes of brain injury. Every year, more than 50 million people worldwide experience a TBI.
A TBI is classified as a blow to the head which disrupts the normal functioning of the brain.
Previous studies have been conflicting, because of small sample sizes and short follow-up periods. Of those, 5.3 percent had sustained at least one TBI during the observation period, which began in 1977.
According to Fann, most people who sustain a single concussion do not develop dementia and the findings do not suggest that every person who sustains a severe TBI will develop dementia later in life. Between 1999 and 2013, 126,734 people (4.5%) aged 50 or older were diagnosed with dementia.
The researchers also discovered that a brain injury in your 20s increases the risk of developing dementia in your 50s by 60 percent.
"TBI was associated with an increased risk of dementia both compared with people without a history of TBI and with people with non-TBI trauma", the authors write.
Men with TBI histories developed dementia at a 30 percent rate, while women had just a 19 percent rate.
The authors called for heightened efforts to prevent TBI, especially among younger people, and said strategies are needed to ameliorate the risk and impact of dementia associated with TBI. They also note that they did not include TBIs treated by general practitioners, so the data might not have captured some less severe TBIs. "The association of TBI with different causes and how these change across time needs policy attention, as it is likely that prevention needs to be considered at societal, community, and local levels".