Apparently some recent research has found that a specific type of tannin (called proanthocyanidin) found only in cranberries and blueberries interacts with the little projections (called fimbriae) on the Escherichia coli (E. coli) bacteria (the most common cause of UTI), preventing them from sticking to the walls of the bladder and causing infection. That means it will help prevent infections from starting, but won’t get rid of an existing infection. For that you need antibiotics.
Apparently the beneficial bacteria lack fimbriae and so are not affected by cranberry juice. However, the bacteria that cause dental plaque do have fimbriae and so are affected by the juice (as long as it isn’t full of sugar).
Unsweetened or lightly sweetened pure cranberry juice is most recommended, and you need to drink about 300–500 ml daily.
Other benefits of cranberry juice are listed here including the fact that it contains Vitamin C, cranberries calcium, folate, iron, magnesium, manganese, phosphorus, potassium, selenium, sulfur, vitamin A, vitamin B-1, Vitamin B-2, vitamin B-3, vitamin B-5, vitamin E, resveratrol, v arious beneficial phytochemicals, and zinc.