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.
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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.