DH was plagued by cold-sores. Years ago, as a teen, a physician friend of his family told him to take the L-lysine EVERY day. He does, and it REALLY works. Now, he only very very occasionally gets one.
The best thing is Zovirax ointment (by prescription) This is expensive, but it really does work!!!
Over the counter, abreva is supposed to help, but it will not stop an oncoming cold sore dead in its tracks like Zovirax will.
I would rather use a topical anti-viral than take one orally, like Valtrex. Only if this were a big problem would I want to do that.
The key to ANY treatment is to start AT THE FIRST TINGLE!!! Once these things really get going. I don't think anything can stop them.
Also, related. Mouth sores, or canker sores inside the mouth. I have had these. Boy are they painfull!!! I found that using a toothpaste high in baking soda has completely eliminated them! Or even a toothpaste/gel with the baking soda along with Peroxide. Right now I am using Arm and Hammer 'Brilliant Sparkle'. Another good one is Mentadent. Discovering this has been a lifesaver! And I wanted to pass the word along.