Drinking too much cranberry juice can cause a mild case of upset stomach and possibly diarrhea. Drinking over 1 liter a day for a prolonged period can sometimes lead to kidney stones. Since some cranberry juices are sweetened with sugar, it's important for diabetics to find juices that use artificial sweeteners instead, according to RxList.
Drinking cranberry juice is an excellent way to keep bacteria from growing in the urinary tract. Since cranberries contain salicylic acid, it's important for anyone to avoid drinking cranberry juice in large amounts if they have an allergy to aspirin, because salicylic acid is similar to aspirin. For preventing urinary tract infections, it is recommended to drink approximately 1 to 10 ounces per day.