When I was a child my mum drank about a dozen cups of coffee a day. She was totally, physiologically addicted to caffeine. So much so that, on Yom Kippur (when no food or drink of any kind is allowed for 25 hours) she would make a paste of instant coffee and a spoonful of water and take it like medicine, just to avoid the crippling caffeine-withdrawal headaches. Looking back on those days it is so strange to me that she is now completely unable to tolerate caffeine at all. Even a cup of tea will keep her awake half the night.