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.

What Is Cranberry Used for Today?

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.1-6,23 If 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

What Is the Scientific Evidence for Cranberry?
Bladder Infection
Prevention

Probably the best evidence for the use of cranberry juice for preventing bladder infections comes from a 1-year, double-blind, placebo-controlled study of 150 sexually active women that compared placebo against both cranberry juice (8 ounces, 3 times daily) and cranberry tablets.9 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.27 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.

A year-long open trial 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.12 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.34 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).38 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.39

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).11,21,28 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.36

Treatment

Another double-blind study evaluated cranberry juice cocktail for the treatment of chronic bladder infections.10  This trial followed 153 women with an average age of 78.5 years for a period of 6 months. Many women of this age group have what are called chronic asymptomatic bladder infections: signs of bacteria in the urine without any symptoms. Half of the participants were given 10 ounces a day of a standard commercial cranberry cocktail drink, the other a placebo drink prepared to look and taste the same. Both treatments contained the same amount of vitamin C to eliminate the possible antibacterial influence of that supplement. Despite the weak preparation of cranberry used, the results showed a 58% decrease in the incidence of bacteria and white blood cells in the urine.

Ulcers

The bacteria Helicobacter pylori plays a major role in the initiation and maintenance of peptic ulcers, ulcers in the stomach and duodenum. A 90-day, double-blind, placebo-controlled study performed in China tested the effects of daily consumption of cranberry juice in individuals who were chronically infected with helicobacter (but who did not necessarily have ulcers).24 The results indicated that use of cranberry significantly decreased levels of helicobacter in the stomach, presumably by causing some of the detached bacteria to be “washed away.” Another study involving 295 children with helicobacter but without ulcer symptoms also demonstrated the ability of cranberry to reduce the levels of the stomach bacteria.35

While this was a promising finding on a theoretical level, it did not directly address treatment or prevention of ulcers. A more practical study evaluated the use of cranberry as a support to standard therapy.33 This double-blind trial enrolled 177 people with ulcers who were undergoing treatment with a common triple-drug therapy used to eradicate H. pylori (omeprazole, amoxicillin and clarithromycin, known as OAC). All participants received this drug treatment for one week. During this week and for two weeks after, they were additionally given either placebo or cranberry juice. Researchers also looked at a third group attending the same clinic, who received only OAC.

The results were somewhat promising. In the study group at large, OAC plus cranberry was no more effective than OAC plus placebo or OAC alone. However, among female participants in the study, use of cranberry was associated with a significantly increased rate of helicobacter eradication as compared to placebo or no treatment.

Does this mean that women undergoing ulcer treatment may benefit from cranberry? Perhaps, but not necessarily. When a treatment fails to produce benefit in the entire group studied, researchers may, after the fact, go on a hunt for a subgroup who did benefit. The laws of chance alone ensure that they can almost always find one. Therefore, it is not clear whether cranberry actually did provide benefit, or whether this finding was merely a statistical fluke.

Dosage

The usual dosage of dry cranberry juice extract is 300 mg to 400 mg twice daily or 8 to 16 ounces daily of pure cranberry juice (not cranberry juice cocktail.)

Safety Issues

As a widely eaten food, cranberry is thought to have a generally good safety profile.

However, several case reports suggest that cranberry could interact with the drug warfarin (Coumadin), potentially leading to internal bleeding.22, 32 Two formal studies have failed to find evidence of such an interaction,30-31 while a third study did find that cranberry can increase the blood thinning effect of warfarin in healthy male subjects.37 Prudence would suggest caution, especially when consuming more than eight ounces of cranberry juice daily.

In addition, cranberry juice might allow the kidneys to excrete weakly alkaline drugs more rapidly, thereby reducing their effectiveness. This would include many antidepressants and prescription painkillers.

Finally, indirect evidence suggests that regular use of cranberry concentrate tablets might increase risk of kidney stones.14

Interactions You Should Know About

If you are taking:

  • Warfarin (Coumadin): Use of cranberry might lead to excessive bleeding.
  • Weakly alkaline drugs (including many antidepressants and prescription painkillers): Cranberry might decrease their effectiveness.