Racing 92 fly-half Pat Lambie has retired from rugby because of "persistent post-concussion symptoms".
The South Africa international, 28, said he had contemplated retiring even before arriving at the French side.
In May 2017, he spent six months out after colliding with a team-mate at Sharks during a Super Rugby clash.
In December that year, he suffered a further concussion in a match at Oyonnax with symptoms "lasting about 40 days", he told L'Equipe.
He returned in February 2018 and played in the Champions Cup final in May but went off with a knee injury as Racing lost to Leinster.
But he said the concussion symptoms grew worse, telling the French sports daily paper he suffered "trembling legs", headaches, migraines and eye irritations.
He had been expected to return this month but said he decided to end his career because of the "persistent post-concussion symptoms".
"I have not been able to do any weight sessions in the last two and a half months because I had a terrible headache," he said.
"I followed the advice of two neurologists. They advised me to stop rugby."
He also spent several months out of the game after a knock to the head in a collision with Ireland flanker CJ Stander in a Test in 2016.
Lambie, who also played at full-back, scored 153 points in 56 Test appearances for the Springboks between 2010 and 2016 and joined Racing in September 2017.