Studies on the effectiveness of cranberry juice for preventing or treating urinary tract infections (UTIs) have produced mixed results. The red berry contains a tannin that prevents E. coli bacteria - the most common cause of urinary tract infections - from sticking to the walls of the bladder, where they can cause infection. However, a 2012 review of 24 studies looking into the effectiveness of cranberry juice and extracts on UTIs found they did not significantly reduce the incidence of the infections.