This FAQ is dedicated to coffee and all that goes with it. There are several newsgroups in which these topics may be of relevance, including rec.food.drink.coffee, alt.coffee, alt.food.coffee and alt.drugs.caffeine. I welcome any and all contributions to this FAQ. If you do not agree with the info in here please let me know or write an article for the FAQ. If you feel you can explain something better than I have, by all means rewrite the article and send it in.
This FAQ file is a collaboration of information from numerous sources. I have made an effort to corroborate as much of this info as is possible but some info may be incorrect or misleading. If you find such info please let me know and I will do further research to find the truth. For this and other reasons please understand that I am not advising you to take any action based on this file. In the realm of medicine I am specifically recommending seeking the advice of a competent medical care giver. I am not giving medical or related advice.
Regular caffeine consumption reduces sensitivity to caffeine. When caffeine intake is reduced, the body becomes oversensitive to adenosine. In response to this oversensitiveness, blood pressure drops dramatically, causing an excess of blood in the head (though not necessarily on the brain), leading to a headache.