Ulcers and sores are common signs of mouth cancer. These sores are abnormal, and they can be present on your lips, inside of your cheeks, throat and gums. Sores may be large or small. One of the most important things to pay attention to is the time it takes for your mouth sores to heal. If you have an ulcer in your mouth that is not healing, you should be concerned. Monitor the ulcer and schedule an appointment with your doctor. Do not ignore the presence of mouth sores and ulcers. They are early warning signs that occur when mouth cancer is highly treatable.