Cranberry juice is commonly used to prevent bladder infections as well as to overcome low-level chronic infections. The cranberry plant is a close relative of the common blueberry. Native Americans used it both as food and as a treatment for 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," and to remove "blood toxins."
In the 1920s, researchers observed that drinking cranberry juice makes the urine more acidic. Because common urine infection bacteria, such as
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 bladder infections. Cranberry fell out of favor after World War II, only to return in the 1960s as a self-treatment for bladder infections.
More recent research has revised the conclusions reached by scientists in the 1920s. It appears that cranberry's acidification of the urine is not likely to play an important role in the treatment of bladder infections; current research has instead focused on cranberry's apparent ability to interfere with the bacteria establishing a foothold on the bladder wall.
If the bacteria can't hold on, they will be washed out with the stream of urine. Studies suggest that in women who frequently develop bladder infections, bacteria have an especially easy time holding on to the bladder wall.
Thus, when taken regularly, cranberry juice might fix this problem and break the cycle of repeated infection.
The best evidence for the use of cranberry juice for preventing bladder infections comes from a 1-year,
study of 150 sexually active women that compared placebo against both cranberry juice (8 ounces 3 times daily) and cranberry tablets.
The results showed that both forms of cranberry significantly reduced the number of episodes of bladder infections.
A double-blind study of 376 hospitalized seniors attempted to determine whether a low dose of cranberry juice cocktail would help prevent acute infections.
It failed to find benefit, most likely due to the minimal dosage of cranberry: only 10 ounces daily of cranberry juice
Furthermore, because of the low rate of infections, it would necessarily have been more difficult for this study to produce
In addition, a year-long
of 150 women found that regular use of a cranberry juice/lingonberry combination reduced the rate of urinary tract infection as compared to a probiotic drink or no treatment.
However, because this study was not double-blind, the results are unreliable. (For more information on why double-blind studies are so important, see
Why Does This Database Rely on Double-blind Studies?
A review of 10 studies investigated the benefits of cranberry juice or tablets compared to a placebo control in patients susceptible to urinary tract infections. Among 1,049 participants, the researchers found the cranberry products reduced the incidence of urinary tract infections by 35%, a statistically significant amount, over a 12-month period. The effect was most notable in those with recurrent infections. However, many subjects dropped out of the studies early, suggesting that continuous consumption of cranberries is not well tolerated.
In another review, researchers analyzed 13 randomized trials involving 1,616 people prone to urinary tract infections (eg, elderly people, people with bladder problems, pregnant women).
In 9 out of the 13 trials, the subjects who consumed cranberry-containing products experienced a decrease in the incidents of urinary tract infections. Another review of 16 randomized or quasi-randomized trials with 3,109 people prone to urinary tract infection showed cranberry juice, tablets, or capsules did not significantly reduce the risk of symptomatic urinary tract infection compared to control (placebo, water, or no treatment), antibiotics, or methenamine hippurate.
On the negative side, three double-blind, placebo-controlled studies failed to find cranberry extract helpful for preventing bladder infection in people with bladder paralysis (neurogenic bladder).
However, a subsequent study of 47 patients with neurogenic bladder from spinal cord injuries found that the use of cranberry extract tablets over 6 months significantly reduced the risk of urinary tract infection.