I had a root canal on one of my front teeth, and had it internally bleached 4 times. It was fine when it was filled, etc., no crown, but now (just under 3 months later) it has started to become sensitive to cold, hot, extreme pressure (the other front tooth isn't) and tapping. It's not so bad that it's unbearable, but it can be pretty uncomfortable, and is gradually getting worse. It is starting now to be very mildly hurting almost constantly.