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A: Oral herpes virus infection usually occurs on the lips, the skin around the lips, or just inside the lips. Rarely it can affect the gums or tip of the tongue. If you have blisters in the back of your mouth or tongue or the roof of your mouth, they are more likely caused by a "summertime" virus called Coxsackie virus; this virus can also cause a sore throat. Rarely, drug reactions or allergies can cause blisters inside the mouth also. If these sores last more than a few days or are getting worse,I would have your doctor check them out.