“This patient was drinking 16 8-ounce glasses of iced tea per day for an unknown period of time. This created a load of oxalic acid that his kidneys couldn't handle, leading to renal failure.” Black tea is rich in oxalate, a compound found naturally in many foods. Too much of it can also lead to kidney stones.
Though moderate intake is healthy for most people, drinking too much could lead to negative side effects, such as anxiety, headaches, digestive issues, and disrupted sleep patterns. Most people can drink 3–4 cups (710–950 ml) of tea daily without adverse effects, but some may experience side effects at lower doses.