What they have advised is for "apthous ulcers" or "canker sores". Its an auto immune condition where the basal cells are attacked and the top layers of the mucosa gets necrosed. Typically, you will have yellowish  lesion with red halo around it.

Its believed that apthous ulcers may be precipitated by acidic food like pineapple, tomatoes and oranges.

Stress is also considered a precipitating factor.

There is no cure - just the symptomatic treatments are available.

If you treat the lesions, they get healed in about a week, if not, they get healed in about seven days.