You may be surprised at how much you know. Caffeine is a stimulant. As a stimulant, too much caffeine can have many side effects.
Excessive intake can cause restlessness, insomnia, heart irregularities, and delirium.
In moderation, caffeine is a mild stimulant that increases urination and the heart's rate. Decaffeinated coffee and teas have no caffeine whatsoever. Even though these drinks should have no caffeine, most contain at least a small amount. This is due to the difficult process required to produce decaffeinated drinks. Usually the small amount present in any of these items will not cause any side effects.