Warning: high consumption of black tea may cause a renal failure
A recent study, published in NEJM, a physician warned about the dangerous effects of black tea on kidney. Dr. Syed received a patient suffering from a renal failure, apparently caused by the accumulation of "Calcium oxalate crystals".
Source: A case of Iced-Tea Nephropathy, Syed F et al. N Engl J Med. 2015 Apr 2;372(14):1377-1378.