CHICAGO, Nov. 18 - College football players who sustain concussions are left prone to another one, especially if they return to action too soon. They also become slower to recover from such blows to the head, researchers say.

InsertArt(2073092)THE RESEARCH — DESIGNED to help schools decide when and if to play injured athletes — support guidelines that say athletes who have had a concussion should wait seven days after symptoms disappear to get back in the game.

The results add to previous research suggesting concussions might make athletes prone to more lasting head injury from another blow.

Some smaller studies have also suggested one concussion might make an athlete more likely to sustain a second one. But this study found the reason may have nothing to do with the athlete’s position or playing style.

Instead, the findings suggest that one concussion might cause tissue injury that leaves players more vulnerable to additional concussions, said Kevin Guskiewicz, director of the sports medicine research laboratory at the University of North Carolina at Chapel Hill.

Multiple concussions are known to increase the risk of permanent brain injury, and Guskiewicz said after three or more concussions it “might be time to think about taking up tennis or golf.”

Concussions are a blow to the head that jostles the brain. Symptoms can include confusion, loss of consciousness, headaches and nausea.

Several pro football players have ended their careers early after sustaining multiple concussions, including quarterbacks Troy Aikman of the Dallas Cowboys and Steve Young of the San Francisco 49ers.

The latest findings were published in Wednesday’s Journal of the American Medical Association.

The findings are contained in two related studies of a total of 2,905 players at 25 U.S. colleges from 1999 to 2001. Guskiewicz helped conduct the studies, which were funded in part by the National Collegiate Athletic Association.

In one study, players with three or more concussions within seven years of the study period were three times more likely to sustain a repeat concussion than players with no concussions. And 30 percent of players with three or more concussions had symptoms lasting more than a week, compared with 7 percent of players with a first concussion.

Ninety-two percent of repeat concussions occurred within 10 days of the first head injury, and 75 percent occurred within a week, Guskiewicz said.

About 34 percent of college football players reportedly have had one concussion, and 20 percent have had more than one. In the NCAA alone, which represents 1,200 colleges and universities, there were nearly 58,000 football players in the 2001-02 season, the most recent data available, said David Klossner, NCAA assistant director of education outreach.

An NCAA sports medicine handbook says players knocked unconscious should not return to play that same day, and those who have had a concussion but no loss of consciousness should be cleared by a doctor to resume playing, Klossner said.

Otherwise, the NCAA has no specific conference-wide restrictions and leaves it up to schools to decide when to allow a concussion-injured athlete to resume playing football, he said.

A seven-day waiting period has been recommended by some medical groups, including the American Academy of Neurology, but doctors and athletic directors do not always heed the recommendation, Guskiewicz said.

Dr. James Kelly, a co-author of the JAMA studies, said thorough mental-function tests should be given to all athletes after a concussion.

Klossner said an NCAA committee on safety will review the data to help provide guidance for colleges, but that decisions on how to handle concussions should be made on an individual basis.

“We can’t make that decision for them,” Klossner said.

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