It can cause brain damage. Old boxers used to have major issues with speaking, thinking, remembering, etc. You know, the shit you never see on talmudvision. Some still do have issues, but it's mitigated somewhat now with modern medical tech and with how much they can get paid they don't need to do too many fights. You might be thinking of lead/heavy metal poisoning. Though I know of some vets with cte that suddenly have anger issues, though that might be a combo of things as well as brain damage.