Docosanol
"I am currently using Abreva for the first time and I will absolutely use it again! I did not apply until after the blister appeared. It was a large cluster sores (about the size of a dime). I am so happy with how fast it worked! Typically, my cold sores last 10 days. It's been 4 and my cold sore has significantly reduced in size - its practically gone. I'm confident it'll be gone by day 7. There has been no crusty dry stage - which I feel is the most embarrassing. I have to agree with another comment I read about the bright white color.  I would have liked to use it more frequently throughout the day, but didn't want to draw anymore attention with white paste all over my face at work. It was worth the $20, a little goes a long way!"