They are rarely contagious as most mouth ulcers are not caused by an infection. The most common cause is a minor accident such as biting on the inside of your cheek, eating something very sharp or brushing your teeth too hard. Most of us have had a mishap like this, and in such cases the ulcer will quickly clear up on its own.

If you are prone to mouth ulcers over a long period, you may be having a reaction to a food - studies suggest that chocolate and coffee can cause problems in some people if taken in large amounts, and so can some kinds of nuts. There is also a lot of evidence that mouth ulcers, like other ulcers, can be caused by stress and anxiety.

Very occasionally, a mouth ulcer is a sign of an underlying illness or a vitamin deficiency, so if you seem to be getting a lot of them it's a good idea to ask your doctor. You can read more about the possible causes here.