Americans consume gallons of coffee, tea, soft drinks and other caffeinated beverages each day. Caffeine is widely used, inexpensive and readily available; it’s found in more than 60 plants and provides the kick in many foods and drinks. But the bitter substance is also an addictive drug.

When you chug down a caffeinated drink, how it affects you depends on the amount of the stimulant you regularly consume, your size, diet and type of exercise you do.

In general, caffeine works by stimulating the central nervous system to increase your heart rate and boost energy and alertness. But caffeine may also cause side effects such as headaches, anxiety, dizziness, jitters and heart problems, especially in those sensitive to the substance.

“For healthy adults, the [Food and Drug Administration, or FDA] has cited 400 milligrams a day—that’s about four or five cups of coffee—as an amount not generally associated with dangerous, negative effects,” explains Michael R. Taylor, deputy commissioner for foods and veterinary medicine at the FDA.

As such, the FDA discourages consuming more than 400 milligrams of caffeine a day.