Pain on one side of the cheek when the mouth is open may be caused by otitis media and mouth ulcers. 1. Otitis media: generally due to bacterial invasion, or by the nose or nasopharynx acute inflammation caused by the spread of inflammation through the Eustachian tube, manifested as ear pain, ear canal water, hearing loss, tinnitus, cheek pain and other symptoms, you can use ofloxacin ear drops, dexamethasone, prednisone, and other medications for treatment. 2. Mouth ulcers: generally due to the lips, palate and tongue and cheeks and other parts of the mouth on the mucous membrane, presenting round or oval painful ulcers, manifested as congestion, redness, swelling, pain, low-grade fever, cheek pain and other symptoms, you can use chlorhexidine gargle, cedirizine tablets, mouth ulcers, ice boron, rehabilitation of the new liquid and other drugs for treatment. There may be other reasons for the pain on one side of the cheek when the mouth is open, so you need to go to the hospital in time to confirm the diagnosis and standardize the treatment under the guidance of the doctor, and the use of medication should be in accordance with the doctor’s instructions.