What's the cause of recurrent mouth ulcers? sometimes one after another for a few months. could be one or a few at a time. sometimes no ulcers.

Several possibilities: Frequent, recurrent mouth ulcers can be a sign of Crohn's Disease. A gastroenterologist should be able to confirm or deny this diagnosis. An allergic reaction to certain foods and/or chemicals in your toothpaste can cause this for some people as well.

No one knows: what causes canker sores. Most mouth ulcers, infections will clear up by themselves within a week or two. You should see your GP or dentist if the ulcer gets worse or lasts for longer than 2 weeks. If concerned see your dentist ASAP.