DD2 keeps getting mouth ulcers. Why and what to do about them?

[Pennies]

DD2 (3.7yrs) keeps getting mouth ulcers. I've taken her to the GP who didn't have anything to say on the subject really.

Does anyone know why some people get them? What causes them? What can I do to try and prevent them? If I can't prevent them what is best to heal them?

She eats well and is otherwise healthy.

I'm interested in anything that can help, be it medical / homeopathic etc etc.

I don't know what causes them, I think it can be being run down, vitamin C might help I think.

I get them badly and I use adcortyl in oralbase, it's a bit thick and pasty but it it the only thing that seems to make them go away.