The right cheek hurts when you open your mouth for a variety of reasons, generally consider oral ulcers, mumps, temporomandibular joint disorder syndrome, local trauma and other causes. 1. Mouth ulcers: mostly due to inattention to dietary habits and habits of life, if the mouth ulcers appear in the mucous membrane lining of the mouth, and mouth ulcers are large, can lead to the right side of the cheeks as soon as you open your mouth pain, can be accompanied by eating difficulties and other symptoms. 2. Mumps: caused by viral infection, it can be manifested as downward and inward swelling centered on the earlobe, pain when opening the mouth, accompanied by fever, difficulty in eating and other symptoms. 3. Temporomandibular Joint Disorder Syndrome (TMJDS): mainly manifested by pain in joints and masticatory muscles, popping and murmur during joint movement, abnormal jaw movement, including limited mouth opening and inability to bite normally. 4. Local trauma: for example, the affected area has been subjected to acute trauma such as external impact, sudden biting of hard objects, and excessive mouth opening, which can cause contusion or strain of the right joint, thus resulting in pain in opening the mouth. The symptoms need to go to the hospital, a clear diagnosis of the cause of the targeted treatment, you can not just randomly use drugs.