An ulcer in the mouth can be caused by factors such as fatigue or illness, but it usually goes away on its own. You can apply an over-the-counter ointment like Orabase to cover the sore, which can help before meals. Acetaminophen can also be used to alleviate the pain. Make sure to use a toothbrush with soft bristles to avoid irritating the gums. Improvement should be noticeable within a few days.