There are several reasons for toothache when you eat hard food: 1, suffering from periodontitis, periodontitis patients due to the resorption of the alveolar bone, will lead to the exposure of the root of the tooth, resulting in the clinical crown is getting longer and longer, but the length of the root in the bone is getting shorter and shorter. The tooth will gradually loosen, and there is no way to withstand excessive biting stress, once biting hard food teeth will appear pain. 2, teeth suffering from chronic periapical periodontitis, is due to decay or trauma led to pulpitis, but not after timely root canal treatment. The inflammation in the pulp cavity spreads to the periapical tissues along the apical foramen, causing inflammatory exudate to appear in the apical area, and even granulomas, abscesses and cysts have appeared. When subjected to excessive biting force, the biting stress will stimulate the inflammatory material around the apical area, causing pain in the teeth when biting hard objects. 3. hidden fracture of the tooth, the tooth will also cause severe pain when biting hard food.