http://www.caffeineandmigraine.com/caffeine.htmlSince Lori used such good language I checked this out. Dilation is indeed what my doctor said but perhaps my doctor was just using the easy explanation on me. If you can get through the article linked above it's an interesting discourse on why and how the caffeine works in different applications but deals mostly with migraine. Long story short, and back to the beginning post, it isn't really bizarre to find caffeine in some of our prescriptions.
As a side note, when I found it to be so effective for me was a time in my life when I consumed practically no caffeine at all. I think that is why a cup of coffee worked so well at making my primary medicine work faster. Friends who regularly drank coffee and also had migraines said they didn't notice the effect.