The short-term effects of caffeine are well agreed upon and widely documented. A good summary appears in The Pharmacological Basis of Therapeutics by Dr. J. Murdoch Ritchie. On the positive side, caffeine produces "a more rapid and clearer flow of thought," and allays "drowsiness and fatigue. After taking caffeine one is capable of greater sustained intellectual effort and a more perfect association of ideas. There is also a keener appreciation of sensory stimuli, and motor activity is increased; typists, for example, work faster and with fewer errors."
Such effects are produced by caffeine equivalent to the amount contained in one to two cups of coffee. According to Dr. Ritchie the same dosage stimulates the body in a variety of other ways: heart rate increases, blood vessels dilate; movement of fluid and solid wastes through the body is promoted. All this adds up to the beloved "lift."
On the negative side are the medical descriptions of the familiar "coffee nerves." The heavy coffee drinker may suffer from chronic anxiety, a sort of "coffee come-down," and may be restless and irritable. Insomnia and even twitching muscles and diarrhea may be among the effects. Very large doses of caffeine, the equivalent of about ten cups of strong coffee drunk in a row, produce toxic effects: vomiting, fever, chills, and mental confusion. In enormous doses caffeine is, quite literally, deadly. The lethal dose of caffeine in humans is estimated at about ten grams, or the equivalent of consuming 100 cups of coffee in one sitting.