What would cause a blister to suddenly appear on your inner upper lip?

A blister on the inner upper lip can occur for a variety of reasons that include burns from hot foods and irritation caused by dentures or braces, according to Healthline. Mouth blisters can also caused by vigorous teeth-brushing that irritates the gums.

Cold sores caused by the herpes simplex virus typically begin as mouth blisters, according to MedlinePlus. The blisters can develop outside or inside of the mouth and are usually preceded by a tingling or burning sensation. Additional causes of lip blisters include irritation from chewing tobacco and lip biting. It is important for individuals with mouth blisters or sores to consult with a doctor if symptoms last longer than two weeks.