The possible causes of mouth blisters are herpes simplex, herpes zoster, allergic lipitis, etc., as follows. 1. Herpes simplex: caused by the herpes simplex virus, lip lesions are mostly recurrent damage that occurs after the healing of primary herpes, often in multiple clusters of blisters. 2. Herpes zoster: caused by varicella zoster virus along the unilateral distribution of peripheral nerves of the cluster of small blisters, if there is a secondary infection can be pustules. 3. Allergic lipitis: allergic lipitis caused by toothpaste, lipstick, chewing gum and other allergens may occur blisters, vesicles, ulcers, the surface is often visible pseudomembrane cover. Mouth blisters may have other causes, it is recommended to go to the hospital in time, with the help of the doctor to clarify the cause of targeted treatment.