It's not clear whether lifestyle before and after a head injury is to blame for the increased risk, if the injury itself has lingering effects, or both, researchers say.
"There is evidence in the study that points to lifestyle factors and health before and after the head injury," said lead author Tom McMillan, of the Institute of Health and Wellbeing at the University of Glasgow.
High rates of death in the year following a severe head injury have been well documented, McMillan and his colleagues write in the Journal of Neurology, Neurosurgery and Psychiatry. But little is known about risk of death after mild head injuries and especially over the long term.
"When we started there was very little research published on late outcome after head injury," McMillan told Reuters Health in an email.
His team did a study on disability after mild head injury and noticed a high rate of deaths, which inspired them to look into it further, he said.
Head injuries are extremely common, McMillan and his colleagues write, and 95 percent of such injuries in Europe are considered "mild."
To see how people with mild head injuries fare over the years, the researchers studied medical records for 2,428 adults admitted to hospitals in Glasgow with mild head trauma between February 1995 and February 1996.
They also identified two comparison groups whose members were similar in age and background, but one group was made up of patients hospitalized for something other than a head injury. The other group contained community members who were not hospitalized and had no history of head injury.
The researchers followed all the subjects for 15 years to compare their rates of death and injury.
Overall, they found the group of head injury patients died at a rate of 2.45 percent per year, compared to the community group's rate of 1.34 percent per year. The group with non-head injuries had a mortality rate of 1.96 percent a year.
The differences were more extreme among the younger subjects, with head injury patients who were under 55 at the beginning of the study at four times greater risk of dying than their peers in the community group.
For the under-55 subjects, the death rate was 1.29 percent per year in the head injury group compared to 0.75 percent per year in the hospitalized group and 0.31 percent per year in the community group.
The authors found that the adult head injury patients tended to have less healthy lifestyles both before and after they were hospitalized and many were likely to have a history of other head injuries before and after the beginning of the study period.
McMillan said some of the lifestyle factors seen in the head injury group include habitual heavy drinking and inadequate exercise.
"We also know that having a head injury is itself a risk factor for having a further head injury, and in those in the study who have a repeat head injury there is a greater risk of premature death," he said.
In addition to lifestyle factors, McMillan said there might be chronic brain changes resulting from repeat head injury, but cautioned that his study had no data on that question.
McMillan said that people are likely to recover quickly from symptoms that occur soon after a single mild head injury.
"However they need to be aware of potential long term risks that are associated with lifestyle and also that they are at a greater risk of repeat head injury, which can result in cumulative effects and persisting symptoms," he said.