Go to the doctor. This could be a type of virus and it will not go away without medicine.
Have you ever had cold sores, fever blisters, or canker sores (to rule them out)? Sometimes the inside of the mouth will break out if you are allergic to something you've eaten; it's the body's way of telling you to stay away from that food (of it may be an additive in a food you've ingested) - that's why some people get reactions to an additive in toothpaste. This is probably nothing, but I'd visit a doctor to know for sure. (Mouth cancers usually start as white patches, not blisters.) It will probably happen again, so I'd want to know what I was dealing with and how to take care of it.
I, too, thought of a reaction to toothpaste. However, if this has been troublesome for any length of time at all, you should see a doctor. Skin cancer of the mucous membranes starts out as a whitish patches I have been told.
For me, these were an allergy reaction to food additives and whitening toothpaste. It took a while to develop, but once I changed back to regular toothpaste, they went away. The problem is that if it is an allergy, it may have taken a while to react and it will not clear up immediately. Of course, there may be other causes. I would see a doctor in case it is something serious.
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