Scientific evidence suggests that cranberry juice may impede the growth of bacteria in the urinary tract. This is said to occur 8 hours after the juice has been consumed, according to WebMD. It is therefore a far easier and cheaper way to treat common urinary infections than other procedures.
Consuming cranberry juice appears to help in urinary tract infections because the active compounds pass through the digestive system intact, as described by WebMD.
A study at the Worcester Polytechnic Institute collected urine samples from participants after they had consumed cranberry juice cocktail. They then attempted to grow E.coli strains in the urine samples, finding that the E.coli was unable to adhere to other bacteria in the urine and multiply.