I used oil of oregano (full strength, undiluted) by dropping a drop on a cotton swab and swabbing the fever blister. There was hardly enough to moisten the cold sore (still in the beginning/stinging stage) but within a few minutes it burned like fire. However, by the next morning it was beginning to heal and never did break into the icky blister stage.
I put oil of oregano (one drop) on my fever blister/ cold sore and withing an hour I noticed it was disappearing. I reapplied throughout the day and by the end of the day it was as if I never had one. It had started out that morning as a potentially nasty one - a lot of blisters covering an area about the size of a dime. As you know, what you see in the morning is just the tip of the ice berg as they continue to grow during the first day or so. I also put three or four drops under my tongue. I bought mine from a health food store. Get the kind that is diluted with olive oil not alcohol. Also, there are places on line that sell it cheaper and in a much higher strength. There are even places that sell it undiluted with instructions on how to dilute it with extra virgin olive oil. I've tried all the other cold sore cures. This is the only one that did anything for me.