The pain on the left posterior border of the tongue is considered to be caused by local injury, and it may also be caused by the development of lobar papillitis, oral ulcers and other diseases. 1. Localized injury, if the mucous membrane of the lateral edge of the tongue is irritated during chewing, resulting in localized injury, the pain at the back of the left side of the tongue may occur. 2. Lobular papillitis, patients with lobular papillitis due to inflammatory stimulation of the lobular papillae, which may lead to increased inflammatory secretions, sensitivity increases, and therefore the symptoms of pain at the back of the left side of the tongue. 3. Mouth ulcers, if mouth ulcers have already appeared, the ulcer site is stimulated may produce this symptom. If there are obvious symptoms of pain at the back of the left side of the tongue, it is best to seek medical treatment as soon as possible.