It depends on your weight, diet, medications, and overall health. Up to about 400 milligrams of caffeine a day is OK for most healthy adults. Over 600 milligrams daily is probably too much. But some people are just more sensitive to it. If you notice stomach problems, headaches, muscle twitches, or heart palpitations, it might be time to cut back. Talk to your doctor about how much is safe if you're pregnant or have heart trouble.