WASHINGTON – Retired football players who suffered three or four concussions have twice the risk of later developing clinical depression – a risk that rises with even more injuries, new research says.

It’s the latest finding that suggests what many people consider merely a bang really can have long-term repercussions. Now scientists are beginning intensive studies to pin down just what happens inside the brain when someone has a concussion.

Concussions, a mild brain injury, can be caused by any hard blow or jolt to the head – from whiplash during a car accident to a tumble onto a sidewalk. Each year, an estimated 1.1 million Americans get a concussion.

Pro and amateur athletes are more likely than the average person to have repeated concussions.

That’s a particular problem because concussions can be hard to diagnose – and getting banged around again before full healing can lead to potentially deadly brain swelling.

“I always say, ‘You can ice your ankle but you can’t ice your brain,”‘ says Dr. Julian Bailes of West Virginia University’s School of Medicine. “You don’t send a player who’s still symptomatic back to play.”

Most people fully recover from a concussion. But a fraction suffer lingering, sometimes severe, problems with memory and other functions – and doctors wonder if sufferers of bad or repeated concussions are more prone to neurologic disease later in life.

As a first step in studying that question, Bailes and colleagues from the University of North Carolina’s Center for the Study of Retired Athletes analyzed data from almost 2,500 retired NFL players. Bailes found no link between concussions and later Alzheimer’s disease or strokes, two common worries.

But 263 of the retired players suffered depression. Having three or four concussions meant twice the risk of depression as never-concussed players – and five or more concussions meant a nearly threefold risk.

The study, presented last week at an American Association of Neurological Surgeons meeting, supports earlier research that linked concussions suffered by World War II soldiers to depression decades later.

For better proof, University of Pittsburgh neuropsychologist Mark Lovell now is tracking how NFL and NHL players fare in the years after a concussion.

More intriguing, he’s using advanced MRI scanners on the brains of high school athletes in a study that will rescan up to 250 of them who later have a concussion – providing before-and-after shots that could finally show just what the injury does to delicate brain tissue.

Scientists already know a concussion somehow throws crucial brain-chemical reactions out of whack, but they’re not sure for how long or if that imbalance could cause a chain reaction leading to later problems like depression.

For now, Lovell and other specialists want athletes and their coaches and relatives to start taking concussions more seriously.

“If you get hit and have a headache, don’t do the macho routine – you need tell somebody,” Lovell says.

Rest is the only way to heal, and Lovell’s research suggests that takes about a week.