This is what happened to me: I used to have so often cold sores that I was desperate. Is hard to believe, but I was getting cold sores even when I looked to somebody else with cold sores. Was absolutely crazy. And one day, many years back, when I was having cold sore I eat a grapefruit and I noticed that instead to grow, started to get dry. I was not 100% sure if was from the grapefruit. Was kind of strange. Usually I had my cold sores 2 up to 3 weeks (so bad they were). In the same time I did nothing unusual beside eating the grapefruit. So, I eat another one and I made sure that I touch the lips with the juice. In 2 days was gone. After that every time when I felt a cold sore is developing I was eating a grapefruit and the next day was gone. What is interesting is that now I do not get them anymore. I tested on somebody else also. I worked with a lady she used to have them very often. I told her about my discovery, she has been using since. She also does not get them very often now, but she still does.
I have to mention that mold in food or in the house can give cold sores. Maybe she has mold in the house that she still get them or in my case was not the house.
To go further, I tried with lemon and vit C. Lemon does not work so well as grapefruit and vit C is better then lemon, but the best I found is the GRAPEFRUIT.
And thank you to EC and all the people who share there knowledge.