One study found 98 percent of people in North America consume some kind of caffeine every day. Caffeine is a natural stimulant that comes in all shapes and sizes, including coffee, tea, chocolate, soda and energy drinks. Moderate doses -- about 200 mg, or two cups of (strong) coffee -- can increase alertness, fight off headaches and may even help prevent Alzheimer's and other diseases. But more than 500 mg per day can lead to addiction, anxiety, irritability and hallucinations. Get the facts before drinking that eighth cup o’ Joe.