Caffeine is a bitter substance found in coffee, tea, soft drinks, chocolate, some nuts and certain medicines. It has many effects on the body's metabolism, including stimulating the central nervous system. This can make you more alert and give you a boost of energy.

For most people, the amount of caffeine in two to four cups of coffee a day is not harmful. However, too much caffeine can make you restless, anxious and irritable. It may also keep you from sleeping well and cause headaches, abnormal heart rhythms or other problems. If you stop using caffeine, you could get withdrawal symptoms.

Some people are more sensitive to the effects of caffeine than others. They should limit their use of caffeine. So should pregnant and nursing women. Certain drugs and supplements may interact with caffeine. If you have questions about whether caffeine is safe for you, talk with your health care provider.