A single TBI characterised as "severe" increased the risk by 35 per cent; a single "mild" TBI or concussion increased the risk by 17 per cent, according to the study published in The Lancet Psychiatry journal.
The research encompassed a large study population, 36 years of follow-up, and access to a uniform healthcare system that tracks the number and severity of TBIs.
Among the nearly 2.8 million people observed, 4.7 per cent had at least one TBI diagnosis.
In the first TBI diagnoses, 85 per cent had been characterised as mild and 15 per cent had been characterised as severe or skull fracture.
The study found that the risk of dementia increased 33 per cent higher for two or three TBIs, 61 per cent higher for four TBIs, and 183 per cent higher for five or more TBIs.
Fann said another important finding is that if you have a brain injury in your 20s, the risk of developing dementia in your 50s is increased by 60 per cent.
"Severe TBI is particularly frequent in young people, and it is concerning that the risk of dementia is particular high in relatively young persons who suffer TBI," said Jakob Christensen, an associate professor at Aarhus University Hospital in Denmark.
Dementia affects 47 million people worldwide, a number expected to double in the next 20 years, researchers said.
Every year, over 50 million people worldwide experience a TBI, which occurs when an external force disrupts the brain's normal function.
Leading causes include falls, motor vehicle accidents, and assaults.
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