CHICAGO >> Jim McMahon would leave home and forget how to get back.

Sometimes, he would stay in his room and lie on his back in the dark because the pain in his head was so excruciating. At his darkest moments a few years ago, when it was just about too much to handle, the former Chicago Bears quarterback thought about killing himself.

“I am glad I don’t have any weapons in my house or else I am pretty sure I wouldn’t be here,” McMahon said. “It got to be that bad.”

McMahon opened up about his struggles with early onset dementia and depression in a gathering with a small group of reporters on Tuesday, issues he believes were brought on by the beating he absorbed playing football. He is scheduled to be honored Wednesday in Chicago by the Sports Legacy Institute, a Boston University-based group that has been studying the effects of brain trauma in athletes and others.

While his suicidal thoughts are a thing of the past thanks to treatment that drains spinal fluid from his brain, the fight with dementia continues. The “punky QB” who once helped the Bears shuffle their way to a championship is also digging in for another battle, one that could have major consequences for the NFL.

McMahon is one of several players identified by name in a federal lawsuit filed in California last month accusing teams of illegally dispensing powerful narcotics and other drugs to keep players on the field without regard for their long-term health.

He also is part of a class-action lawsuit in which the NFL agreed to a $765 million settlement without acknowledging it hid the risks of concussions from former players. A federal judge has yet to approve the settlement, expressing concern the amount is too small.

While McMahon wouldn’t discuss the most recent suit, he did talk about the troubles he has faced in recent years, issues he believes took root when he was getting battered on the field.

McMahon said he had three to five diagnosed concussions and who knows how many more that went undiagnosed. That’s in addition injuries to the kidney, broken ribs, an addiction to painkillers and a broken neck that he said team doctors and trainers never told him about.

He found out about five years ago, when he went for X-rays and an MRI. Doctors told him he had broken his neck at some point, and McMahon believes it happened with the Minnesota Vikings during the 1993 season, when he got sandwiched by two Giants defenders in a playoff game at New York.

Nearly broken in two, McMahon couldn’t move his legs at first. He eventually headed to the sideline after about 10 minutes. He didn’t stay there long.

He said he went back in — “like an idiot” — and a defender trying to block a pass grazed his head. McMahon’s legs went numb again and he left the game.

McMahon said the doctor asked him afterward how he felt but did not examine him. He said there’s “no doubt about it,” the team knew his neck was broken.

A message was left Tuesday seeking comment from the Vikings.

The dementia diagnosis came five years ago, after McMahon was having trouble remembering the most basic stuff.

He would meet someone and forget their name. For that matter, he had trouble remembering the names of people he knew for years.

That wasn’t all.

He’d go out and forget the way home, so he would call his girlfriend Laurie Navon and tell her: “I don’t know where I’m at. I don’t know how I got on this road. I told her, ‘Aliens abducted me and put me over here.'”

Navon mentioned the mood swings, seeing a man who was “mad at himself, mad at the world.”

He was suffering, with excruciating pain in his head. It was so bad he would hole up in the dark in his room for weeks, only leaving if he had an appearance to make.

“I can see how some of these guys have ended their lives, because of the pain,” McMahon said.

Lately, he’s been getting relief from two doctors in New York. Through a machine they invented, they’re able to relieve the pain in his head through a nonsurgical procedure that realigns his neck every few months.

Spinal fluid cooling in the brain brought on by a rotation in his vertebrae was causing the headaches. By realigning the neck, the fluid drains. In turn, the pain goes away for a few months, along with the short-term memory loss and moodiness.

The dementia, however, is an ongoing fight, one of several for a former quarterback going nose to nose with the league.

“The NFL continues to make billions and billions of dollars every year,” McMahon said. “And some of these guys are homeless. They don’t know who they are, and they were the ones who built this brand to where it’s at.”