There are several different types of mouth sores and they can be pesky and bothersome. Unless a mouth sore lasts more than two weeks, it is usually nothing to worry about and will disappear on its own.
Common mouth sores are canker sores (aphthous ulcers) that occur inside the mouth and not on the lips. They are not contagious and can be triggered by many different causes. They are only a concern if they don’t go away after two weeks.
Fever blisters or cold sores are caused by the Herpes simplex virus and occur on the edge of the outer lips. They are contagious and will come and go but are not completely curable. Mouth sores are also seen in oral thrush or Candidiasis, a yeast infection of the mouth that can be seen in infants, denture wearers, people with diabetes, and during cancer treatment.