What causes blisters inside the mouth?

Possibly herpes. Possibly oral herpes (hsv) also known as a fever blister. Another possibility is a canker sore which is different from hsv. Hsv should be treated with an oral antiviral such as acyclovir, valtrex, (valacyclovir) or famvir. Hsv is contagious and there is no cure, it can keep recurring and you are always a carrier. Canker sores are not contagious.