There are several reasons for the tooth filling to fall off in a few days: 1.It may be that the tooth decaying tissue is not completely removed during the process of filling the cavity, so that the filling material is not directly bonded with the healthy tooth tissue, thus causing the filling material to fall off easily. In this case, when you want to fill again, you need to completely remove the infected and decayed tissues before filling the cavity. 2.The occlusal high point of the filling material, because the filling material is not carefully adjusted after the filling, the occlusal high point of the surface of the filling material causes the local stress concentration and makes it fall off in a few days after the filling. 3.Choose the glass ion filling material in the process of filling. Because glass ionomer filling materials are not chemically bonded to the dentin, they are easier to fall off. If you choose composite resin for filling again, because of the presence of composite resin bonding agent, there will be a chemical bonding process between the composite resin and the tooth surface, so it will be relatively strong.