With composite fillings sensitivity is usually present, but if it is, it usually is lasts a short period of time. However, 10% of all fillings will need a root canal at some time in the future. Because decay is soft and acts like insulation, your tooth might have been comfortable the day before the filling, but you may still need a root canal if the sensitivity persists longer than usual. If your tooth needs a root canal within 12 months of the time of the filling was placed, we will credit the cost of the filling towards a post, core build-up and crown.