Contrary to popular belief, the cranberry juice commonly found on grocery store shelves is ineffective at preventing urinary tract infections.
Cranberries do contain compounds that defend against bacterial infection in the bladder wall, which can help prevent UTIs, but cranberry juice does not have a high enough concentration of these compounds to do much good. In order for a noticeable reduction in bacterial adhesion, a person would have to consume at least 32 ounces of cranberry juice daily.