Former All Blacks prop Carl Hayman, 41, has joined in legal action against World Rugby, after being diagnosed with early onset dementia.
Hayman joins the litigation, which also includes England’s 2003 World Cup-winning hooker Steve Thompson.
He underwent extensive testing in England, after suffering persistent headaches that led him to alcohol abuse and thoughts of suicide.
"I spent several years thinking I was going crazy," Hayman tells The Bounce. "At one stage, that's genuinely what I thought.
"It was the constant headaches and all these things going on that I couldn’t understand.
"It would be pretty selfish of me to not speak up and talk about my experience, when I could help a guy in New Zealand, perhaps, who doesn’t understand what's happening to him and has no support network to lean on."
Hayman - who played for Newcastle in the English Premiership - and his fellow claimants say World Rugby and England's Rugby Football Union failed to protect them from the risks of concussions and sub-concussions, forcing them to take the field, when they were unfit to do so.
"Basically, if I was fit and available, I was on the field," he tells The Bounce.
"There were times that I probably shouldn't have played, but it was expected - like when I had a root nerve anti-inflammatory injection in my neck during the week and was back on the pitch at the weekend.
"They worked us hard and I never complained. It was my job and I was paid well, but I doubt it did any favours.
"I'm 41, I've still got a massive part of my life ahead of me and when you live with something like this, it certainly makes every day a challenge."
Hayman says his lowest point was being involved in a physical altercation with his ex-wife in an incident that resulted in a domestic violence charge in France. He was handed a four-month suspended prison sentence.
"I don't want to minimise the harm I did and I don't want to make any excuses, because I should never have put myself in that situation, but I have so much regret, because that's just not who I am.
"I'm not an angry person, but I was in a deep, dark place and, unfortunately, I will have to carry that with me forever."
Once considered the world's premier front-rower, Hayman played 81 games for Super Rugby's Highlanders and earned 46 caps for the All Blacks between 2001-07, before heading to England to join Newcastle, then France's Toulon.
He retired in 2015, transitioning to an assistant coaching role with France's Pau, which ended in 2019.
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