cold sores
"I've had cold sores since I was 8. I would get them every 2 months and they would last about 2 weeks before they were completely gone. My dentist prescribed me valtrex. If I take it at the first hour of feeling a cold sore coming on it works great.  I don't get the little blisters that form on the lip, just feels a little swollen.  It goes away after 3 days. 
Unfortunately it doesn't work once the blisters have formed.  So if you get a cold sore in the morning and only get home at 8pm to take the valtrex it won't work the same way.
So I always have it in my purse just in case.  Life saver."