Caffeine is a stimulant that can lead to anxiety and digestive problems if over-consumed. For healthy adults, 200 to 300 milligrams of caffeine a day, or two to three cups, is safe. Common caffeine sources include coffee, soda and tea, including sweet iced tea from fast food restaurants such as McDonald's.
McDonald’s sweet tea is made from black tea, which contains 47 milligrams of caffeine per one cup. A large serving size contains 30 fluid ounces and 180 milligrams of caffeine, which is within the range of moderate caffeine consumption considered safe. A small iced tea, or 16 fluid ounces, has 96 milligrams of caffeine, and a medium, or 21 fluid ounces, has 126 milligrams of caffeine.