According to a study, that has been published by the Journal of American College of Nutrition in 2003, american scientist doubt the diuretic, i.e. the dehydrating effect of caffeinated beverages like coffee.

Also individuals suffering from severe migraine are being adviced to avoid specific meals and beverages such as coffee. As the German Migraine and Headache Association (DMKG) had published, austrian studies verified, that even less coffee consumption can cause migraine!

It has been detected that sudden coffee deprivation can cause headaches for individuals with rather strong coffee consumption. Caffein does have a vasodilatory effect and sudden caffeine insuffience can cause vessel contraction. A typical reason for headache. I would make more sense to reduce coffee consumption continuously.