Abreva
"I get a cold sore every May and December... Last December I used Abreva
" At the first tingle " and it just never developed and was totally healed in 2 days. 
Now it's May and I cut this one off at the pass even sooner but the results are totally opposite. It does seen smaller and less painful then if I didn't use anything but it's still a full blown cold sore.
The jury is still out on this subject. I really can't tell with just 2 uses. But the good news is I have the rest of my life to do more research. I'll update my review next December."