The main causes of monthly toothache are chronic pulpitis, apical periodontitis, and dentin hypersensitivity. Monthly occurrence may be endocrine disorders and other reasons to induce changes in these conditions, and thus the emergence of tooth pain. 1. Pulpitis, apical inflammation: tooth decay triggers deep caries, which remains untreated leading to chronic pulpitis or periapical inflammation. Due to the infection of the pulp nerve, tooth stimulation may cause pain, especially when eating hot, cold, sour and sweet foods, which will cause persistent and severe pain. If the infection has spread to the root tip, which will cause periapical inflammation, biting hard objects will cause swelling and pain in the root tip area. 2. Dentin hypersensitivity: Receding gums or hidden cracks in the teeth cause exposure of dentin, leading to dentin hypersensitivity and causing soreness and pain in the teeth. If the above problems occur repeatedly every month, it is recommended to seek medical examination and treatment in time.