There are various reasons you may have a mouth blister or mouth sore, some more serious than others. These blisters and sores can be caused by mouth trauma (biting, scratching, or damage inside of the mouth), viral infections (such as herpes), underlying conditions like Aphthous Stomatitis (leads to canker sores), allergies and food sensitivities, hormonal fluctuations, diseases (HIV/AIDS, Behcet’s Disease, Crohn’s Disease, etc.), mouth burns, and even high stress. Learn more in our blog article, "Mouth Ulcers & Sores: Symptoms & Treatment."