Professional footballers who have had concussion are more likely to suffer depression and other problems including sleep disturbance when their playing careers end, according to new research.
The study, carried out on behalf of players' union FIFPro, also surveyed ice hockey and rugby players and showed those who had suffered "four of five concussions" were 1.5 times more likely to report mental health problems than those who had not.
Players who had suffered six concussions or more were between up to five times more likely to report mental health issues.
The study questioned almost 600 former footballers, ice hockey and rugby players from eight countries, with all aged under 50.
It said: "These findings mean that, during the first 10 years after retirement, players are seven to 11 percent more likely to report symptoms of common mental disorders for each additional career-related concussion. The mean number of concussions was three per player."
FIFPro chief medical officer Dr Vincent Gouttebarge was among the experts behind the research, which concluded that there was still a need to educate people about what to do when a concussion happens.
He said: "This is an important piece of research that suggests concussion might be a contributor to the mental health problems suffered by many players.
"We as football stakeholders -- federations, clubs and player unions -- need to do be alert to the mental health of players, both during and after their careers.
"That means educating players about the dangers of what can be an intense and stressful career and supporting them when they need assistance."
The report said more support should available for ex-athletes suffering with mental health problems, but stressed there was no suggestion that the mental symptoms indicated any degree of brain damage.
Last month, Colorado Rapids forward Kevin Doyle said he was retiring from professional football because of repeated concussions.