Sores inside the mouth are commonly called canker sores, but only some of them truly are. Mouth ulcers, these painful spots that seem to crop up randomly, can be caused by two things- local trauma (like braces rubbing or biting your cheek), or aphthous stomatitis. Apthous stomatitis is a common, non-severe condition characterized by the regular recurrence of mouth ulcers, called (when they originate from this condition) canker sores.
You might experience more mouth ulcers when you eat sharp foods, like crackers or chips, spicy foods, very hot or cold foods, or acidic foods, as these can all wear away at the mucosal membrane inside your mouth. The causes of canker sores however are much more varied, and worse understood than simple mouth trauma induced sores. It’s believed that canker sores are a result of an T-cell mediated immune response, but what triggers this response seems to vary widely from patient to patient. Stress seems to play a large factor in canker sore outbreaks, but it’s also been suggested that ulcers at times of stress are just due to lip and cheek chewing. Likewise, nutritional deficiency seems to play a role, though it’s not clear how. Some theorists believe that canker sores form as a result of heightened antigenic sensitivity, so that when patients are exposed to bacteria and viruses throughout their lives, their immune systems ‘overreact’ and form a canker sore in the process of trying to kill the bug. This theory has gained traction with the discovery that allergic reactions to foods often involve forming mouth ulcers.
Canker sores are a common symptom of many diseases, such as celiac disease, ulcerative colitis, and HIV/AIDS. Rarely, canker sores can be due to oral cancers, but the majority of the time these sores are simply present on their own, and will heal in 7-10 days. Medication for canker sores and mouth ulcers does exist, but it is largely to lower inflammation or reduce pain, not to ‘cure’ the sore itself.