I find that the best thing is Zovirax cold sore cream. If you can get the cold sore early - when there is just the sens that there is a lump under the skn and you start putting the cream on then, it will very often not even develop into a full cold sore - or at least only a very small one.
Once the sore breaks out then it is a bit late. Zovirax helps to shorten its life but I also use some tincture which you paint on from a little bottle which help to dry it out. Ask at your chemist.

Zovirax was prescription only until a few years ago. Now you can get it over the counter. It's a bit pricey and you only get a tiny tube which I keep on losing. I reckon that I buy a new tube 3 or 4 tmes a year but at least it works.

I supose that you do know that cold sores are some kind of herpes. They are very contagious and the virus lasts for life. Herpes simplex virus - Wikipedia, the free encyclopedia - More enduring than love, they say.