The cranberry plant is a close relative of the common blueberry. Native Americans used it both as food and for the treatment of bladder and kidney diseases. The Pilgrims learned about cranberry from local tribes and quickly adopted it for their own use. Subsequent physicians used it for bladder infections, for "bladder gravel" (small bladder stones), and to remove "blood toxins."

In the 1920s, researchers observed that drinking cranberry juice makes the urine more acidic. Since common urinary tract-infection bacteria such as E. coli dislike acidic surroundings, physicians concluded that they had discovered a scientific explanation for the traditional uses of cranberry. This discovery led to widespread medical use of cranberry juice for treating bladder infections. Cranberry fell out of favor with physicians after World War II, but it became popular again during the 1960s—as a self-treatment.

Cranberry is widely used today to prevent bladder infections, although as yet the evidence to support this use remains limited. Contrary to the research from the 1920s, it now appears that cranberry's acidification of the urine is not likely to play an important role in the treatment of bladder infections; current study has focused instead on cranberry's apparent ability to block bacteria from adhering to the bladder wall.1-6,23If the bacteria can't hold on, they will be washed out with the stream of urine. Interestingly, studies have found that in women who frequently develop bladder infections, bacteria seem to have a particularly easy time holding on to the bladder wall.7 This suggests that cranberry juice can actually get to the root of their problem.

Just as cranberry seems to prevent adhesion of bacteria to the bladder, preliminary evidence suggests that it might also help prevent the adhesion of the ulcer-causing bacteria Helicobacter pylori to the stomach wall. On this basis, it has been proposed for preventing or treating ulcers, with mixed results as described below.

Other preliminary evidence suggests that the same actions of cranberry juice might make it useful for treating or preventing cavities19 or gum disease.13 However, there is one kink to work out before cranberry could be practical for this purpose: the sweeteners added to cranberry juice aren't good for your teeth, but without them cranberry juice is very bitter.

Cranberry has also been investigated as a possible aid in reducing the risk of heart disease and cancer,16-18,25 and as a treatment for diabetes,20 but there is no meaningful evidence as yet that it is actually helpful for these conditions.

One study failed to find cranberry significantly effective for enhancing mental function.26