How to know when a cold sore is healed?

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Cold sores can usually heal by themselves sometimes within a few days or 2 weeks. But a cold sore can't go away, it's a form of herpes that affects area around the region of the mouth and are infectious and is spread usually if a person touches a cold sore or touches infected fluid—such as from sharing eating utensils or razors, kissing an infected person, or touching that person's saliva. A parent who has a cold sore often spreads the infection to his or her child in this way. Cold sores can also be spread to other areas of the body. But also don't touch the area of a cold sore if you feel one coming in.

Use lemon juice on it. It has worked better than anything else for me. It will sting, but it will be gone by tomorrow most likely. And keep your tongue off of it.