Caffeine is a natural ingredient in coffee, cocoa, tea, and chocolate, and is added to some prescription and non-prescription drugs. Despite being natural, Caffeine is also a powerful drug that greatly affects the body. In healthy, rested people, a dose of 100 milligrams ( about one cup of coffee) increases alertness, banishes drowsiness, quickens reaction time, it enhances intellectual and muscular effort, and increases heart and respiratory rates. Drinking one or two cups of coffee an hour before exercise encourages the body to preserve glycogen and burn fat something that results in greater endurance. In addition, caffeine masks fatigue. In doses above 300 milligrams, caffeine can produce sleeplessness, nervousness, irritability, headaches, heart palpitations, and muscle twitches. Caffeine is also habit-forming and those who try to suddenly stop after heavy use may experience such withdrawal symptoms as headaches, lethargy, irritability, and difficulty in concentrating.