Carlisle, who called a halt to his playing career at the end of last season, is presenting a television documentary this week about suicide and depression among footballers. In an interview with the Mail on Sunday, he talked about his own depression and the time he tried to take his life as a young professional. It is an issue he believes is widespread.
"I will categorically state there are hundreds of players suffering with this," he said. "The numbers in society are one in four and footballers are members of that society."
He added: "The thing about football is that the reluctance to come forward and speak means there are so many guys sitting under the radar.
"As PFA chairman, I've had 15-20 guys come to me and say: 'Clarke, there's something wrong with me and I don't know what it is or where I should go'.
"All they know is they want to be out of football, out of the system. And those are just the ones who had my number and felt they could call. You cannot undersell this."
To help combat the problem, Carlisle plans to launch a hotline for footballers to call if they feel the need to reach out for help.
It comes after Carlisle recalled an occasion when, as a youngster at QPR, he went to a local park with a bottle of pills and attempted to end his life.
"I'd decided that ending my life was the best and most pain-free solution for everyone," he said. "It wasn't a cry for help. I downed the pills and was expecting some really dramatic ending, like a movie scene.
"When that didn't happen, I thought I'd go back to my flat, have another can and go to sleep, job done. It's frightening, really frightening, to think about my state of mind back then."