What are possible reasons for an inner lip blister lasting over two months? (photo)
You most likely have a mucous retention cyst. Mucous retention cysts arise in the minor salivary glands as a result of injury to the gland or its duct. The common sites are on the mucosal aspect of the lower lip. Typically the patient presents with a history of recurrent swelling that develops over days or weeks, ruptures and then recurs after a few weeks.
Definitive treatment involves excising the cyst under local anaesthesia by a dental surgeon.
Hope this helps!