Caffeine is a psychoactive drug, meaning it alters mood, thinking or behaviour. Caffeine speeds up the functions of the central nervous system and stimulates thinking, keeps people awake and suppresses appetite. In large doses, caffeine can produce headaches, jitters, nervousness and even delirium. This is especially so for someone not used to taking caffeine.
Caffeine also stimulates the secretion of gastric acid and increases urination.
Most researchers agree that there is very little health risk when less than 600mg of caffeine is consumed a day. Regular use of more than 600 mg of caffeine per day can cause chronic insomnia, persistent anxiety, depression and stomach upset. People can become dependent on caffeine. This means they have a strong desire to have caffeine because it has become important in their daily lives. If caffeine is unavailable they may panic or feel anxious.