It's something to do with shrinking the blood vessels I think. When you have a headache, your blood vessels are enlarged, caffeine shrinks them, less pressure. Something like that. Caffeine addicts' (like me) are used to small blood vessels, when they don't get any caffeine for a bit, the vessels enlarge again, causing headache. Drink caffeine to get rid of headache, addiction continues...vicious circle I'm currently trapped in lol