Canker sores are sores on the inside of the mouth. They are caused by a virus and can be very painful. The first symptoms can be pain, redness, swelling, or a sore that is filled with fluid. These symptoms usually occur in the mouth, throat, and lips. If you have these symptoms you should see your dentist to diagnose the cause of the problem. There are many treatments for canker sores and there is no cure for them, but they are not life-threatening and will go away on their own.