Cranberry concentrate has not been reported to cause side effects and has no known contraindications to use during pregnancy and breast-feeding. According to one report, supplementation with an unspecified number of cranberry tablets for seven days increased the urinary excretion of oxalate by 43%, suggesting that long term use of cranberry supplements might increase the risk of developing a kidney stone.⁹ On the other hand, in the same study, urinary excretion of magnesium and potassium (which are inhibitors of stone formation) also increased. Until more is known, individuals with a personal or family history of calcium-oxalate kidney stones should consult a doctor before using cranberry supplements for long periods of time (e.g., more than a week). Cranberry should not be used as a substitute for antibiotics during an acute urinary tract infection, except under medical supervision.