Hello, Viral sores can be common and quite painful. If these are caught early with a slight tingle sensation, antiviral prescription medication can lessen the occurrence. Abreva is available over the counter and can offer some relief. Viruses need time for symptoms to reside and often can reoccur due to stress or compromised health. Aphthous ulcers need time to health as well. Warm salt water rinses and avoiding all irritants can be beneficial. Avoid smoking, alcohol, spicy and acidic foods. Keep well hydrated and make sure you do not have any vitamin deficiencies. Follow your treatment recommendations as prescribed for the mononucleosis. See your doctor or dentist for oral symptom prescription relief such as a comfort rinse that can include Antiinflammitory and pain relief medication. A lidocaine rinse can offer a numbing effect. Thank you for your inquiry.