Can cranberry juice fight a uti that is already present, or does it only prevent them?

Weak action: Cranberry juice is a weak UTI preventative and virtually of no use in treating an active uti. Most utis burn themselves out without medication, as used to happen in olden times before the advent of antibiotics, unless there is a structural abnormality of the urinary tract. Drinking plenty of liquids and a short course of antibiotics are best weapons against utis.