To go mad (狂れる): used mainly in the phrase '気がふれる' to mean to lose one's sanity or become mentally unbalanced; somewhat literary/archaic compared with 気が狂う.
彼は深い悲しみに打ちひしがれ、ついに気がふれてしまった。
He was overwhelmed by deep grief and eventually lost his mind.
長年続く隔離生活で気がふれるのではないかと心配だ。
I'm worried that prolonged isolation might drive someone crazy.