Caffeine acts as a stimulant that narrows blood vessels in the brain. When you stop drinking it abruptly, those blood vessels widen rapidly, causing increased blood flow and a throbbing headache. Your body has adapted to the daily intake, creating a mild chemical dependence. To reduce your intake without the pain, you should taper off gradually rather than stopping cold turkey. Start by mixing your regular coffee with a bit of decaf, or reduce your daily cup size by twenty percent every few days. Staying hydrated with plenty of water and eating regular, balanced meals will also help stabilize your energy levels throughout the transition period.