Some studies suggest that cranberries can prevent urinary tract infections, particularly in women who are prone to these infections, according to WebMD. As of 2015, researchers believe cranberries accomplish this with a substance that prevents bacteria from sticking to the walls of the urinary tract.
One study found that women with a history of UTIs caused by E. coli decreased their likelihood of contracting another UTI by 20 percent when consuming a little more than 1.5 ounces of cranberry-lingonberry juice concentrate daily for a six-month period, reports WebMD. In another study, sexually active women either took one concentrated cranberry tablet per day or drank 8 ounces of cranberry juice daily for one year, and both cranberry products were linked to a smaller number of patients who experienced symptomatic UTIs.
Although cranberry products seem to have a beneficial effect for women with a history of UTIs, the effectiveness for children, seniors and other groups is not well supported by research, notes WebMD. Because cranberries do not kill infection-causing bacteria, they are not effective in treating current UTIs. Cranberry juice can also have a negative effect on the urinary tract due to high levels of oxalates, which can crystallize and become oxalate stones if the individual consumes enough of the substance.