There are studies showing that cranberries reduce the recurrence of urinary tract infections in some cases, but there is no evidence that they effectively treat existing UTIs, according to WebMD. Official medical organizations do not believe there is enough evidence to support the use of cranberries for UTI prevention.
For women who are at high risk of recurring UTIs, studies show that there is a preventative benefit to drinking cranberry juice or taking cranberry supplements, says WebMD. However, studies do not demonstrate this benefit for other groups of people. Researchers believe the preventative benefit is due to cranberries containing a substance that prevents E. coli, the bacterium that causes UTIs, from taking hold within the urinary tract.