Caffeine is a common stimulant that acts on the central nervous system. Most people know that caffeine comes in tea, coffee, some fizzy drinks and chocolate, but it’s in many medicines too. It’s also sold in stimulant tablets that some people take to stay awake. Caffeine can both cause and relieve headaches. When in medications, it acts to improve absorption of the other substances in the product, while also causing blood vessels in the brain to constrict slightly, which can relieve headaches caused by dilated veins (eg migraines). However caffeine can also cause headaches; in fact, it’s the caffeine withdrawal that causes the pain. Since caffeine normally induces vasoconstriction of the blood vessels in the brain, a sudden lack of it leads to vasodilation (ie the blood vessels swell). This can give a ‘pounding’ headache, which is relieved as soon as you have some caffeine. It’s not just the heavy coffee drinkers who get this; some people can become ‘addicted’ to very small amounts of daily caffeine.