caffeine is amazingly useful for headaches. I take prescription migraine drugs that also have caffeine in them, it's such a useful additive!

Especially now, at the beginning of LCing, when the headaches are the worst, I say, take the darned stuff if it makes you feel better.

from "How things Work"

"To a nerve cell, caffeine looks like adenosine. Caffeine therefore binds to the adenosine receptor. However, it doesn't slow down the cell's activity like adenosine would. So the cell cannot "see" adenosine anymore because caffeine is taking up all the receptors adenosine binds to. So instead of slowing down because of the adenosine level, the cells speed up. You can see that caffeine also causes the brain's blood vessels to constrict, because it blocks adenosine's ability to open them up. This effect is why some headache medicines like Anacin contain caffeine - if you have a vascular headache, the caffeine will close down the blood vessels and relieve it."