Caffeine is a bitter substance that is found in coffee, tea, chocolate, soft drinks and some medicines. It has many effects on the metabolism of the human being, including that of stimulating the central nervous system making a person feel more alert and having more energy.
For the majority of persons, having between two to four cups of coffee a day is not harmful. However, too much caffeine can make a person restless, anxious and irritable. It can also impede sleep and cause headaches, abnormal cardiac rhythm or other problems. If one has large quantities of caffeine for long periods of time, there may also be withdrawal symptoms when the caffeine intake ceases.