A blister has appeared in my mouth.

Most blisters in the mouth are lesions that heal with time - canker sores, herpetic blisters, burns, or a mucocele - and an ordinary canker sore heals within 1 to 2 weeks. However, 1) an ulcer or blister in the same place that has not healed in 2 to 3 weeks, 2) one that keeps growing or becoming firm, and 3) mucosa that stays white or red have to be distinguished from conditions such as oral cancer, so an examination is essential. If they recur often, irritating factors such as a sharp tooth or restoration should also be checked.