Probably 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; cranberry tablets were more cost-effective.
A double-blind study of 376 hospitalized seniors attempted to determine whether a low dose of cranberry juice (as cranberry juice cocktail, 10 ounces daily—a very low dose compared to the previous study) would help prevent acute infections.
It failed to find benefit, perhaps in part due to the dosage of cranberry, as well as the low number of infections that developed overall.
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 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 incidence 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, 3 other double-blind, placebo-controlled studies evaluated the effectiveness of cranberry extract for eliminating bacteria in the urine of people with bladder paralysis (neurogenic bladder).
The results showed no benefit. 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.