Scotland forward David Denton has decided to end his playing career on the advice of doctors.
The 29-year-old back row has been advised to stop playing following a concussion he sustained playing for Leicester Tigers in October.
"There is more to life and that has become clearer to me," Denton, who was capped 42 times, told BBC Scotland.
The national team's head coach Gregor Townsend said: "We're going to miss him with Scotland."
Zimbabwe-born Denton joined Edinburgh in 2010 and had spells at Bath and Worcester Warriors prior to a switch to Leicester last year.
He had not played for the Tigers since suffering the injury against Northampton 11 months ago.
"I've been in the frame of mind of knowing this was coming for a while," Denton told the John Beattie Show.
"I met with a neurologist three months ago and he alluded that if things didn't change by the next time I saw him then this was probably the way it was going to go.
"I called Gregor [Townsend] just to make it clear I was going to be unavailable for the World Cup and then these things just kept snowballing.
"I felt like I kept pushing it then eventually it got to the point where it became clear it wasn't wise for me to play rugby anymore."
There have been calls to address the issue of concussion in the game, with former Wales captain Sam Warburton asking World Rugby to standardise the way head injuries are treated.
But Denton added: "I understand people being anxious about it. From my point of view, I think World Rugby are doing as much as they can at the moment.
"I don't think there are many rules that you can change that are going to lower the number of head injuries.
"Lowering the tackling height below the shoulders has been a very positive step and I think it's very important to stop ball carriers getting concussions.
"As has been suggested by a few people, I think lowering the tackle height to below the waist will be a mistake.
"I think that would cause more concussions and at the professional level that's something that is definitely not taken lightly. At the junior level it is something that can definitely be looked at."
Scotland head coach Townsend said it "seems to be the right decision to retire".
He said: "We were hoping taking some time out of the game would mean he would be available for selection in our World Cup training squad and when that didn't happen, we were hopeful he'd be back for next season, but again that's not happened."
BBC Scotland's chief sports writer Tom English
Four years ago he was one of the stars of Scotland's World Cup.
He was terrific throughout the pool games, and terrific against Australia in that what-might-have-been quarter-final.
Denton was a big ball-carrying number eight - the kind of player Scotland could do with now. It's a great shame, he's had a lot of injuries.
But, it just adds to the rising number of retirements through concussion. These brain injuries are a serious, serious problem in rugby.