Cranberry juice provides no meaningful protection against cystitis, contrary to the belief of large numbers of women, say scientists.
A review of findings from 24 studies involving 4,473 participants found no evidence that cranberry juice, or supplements, can be used to prevent bladder and kidney infections.
Cystitis is a stinging inflammation of the bladder usually caused by a urinary tract infection (UTI). It can also be triggered by irritation during sexual intercourse, leading to its nickname the "honeymoon disease".
For decades countless women have used cranberry juice to treat mild cystitis or prevent recurring infection.
Experts have suggested that compounds in cranberries may stop bacteria sticking to cells lining the walls of the urinary tract.
Numerous websites and many GPs encourage women troubled by cystitis to try the cranberry treatment on the basis that it might help and will do them no harm.
In 2008, a review of 10 trials found that women who drank the juice or took cranberry supplements had fewer UTIs that those who did not.
Like the latest review, it was published in the Cochrane Library that specialises in assessing medical evidence to inform guidelines and health policy.
Some of the studies looked at in the new review showed "small benefits" for women suffering from recurrent infections.
However, the authors stressed that these were not statistically significant. Women would have to consume two glasses of cranberry juice every day over long periods to prevent one infection, they pointed out.
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Lead researcher Dr Ruth Jepson, from the University of Stirling in Scotland, said: "Now that we've updated our review with more studies, the results suggest that cranberry juice is even less effective at preventing UTIs than was shown in the last update.
"We can't see a particular need for more studies of the effect of cranberry juice, as the majority of existing studies indicate that the benefit is small at best, and the studies have high drop-out rates."
A common problem when evaluating studies of cranberry tablets or capsules was that they rarely reported the level of active ingredients in the supplements, said the authors. It was therefore difficult to judge whether levels would have been high enough to have any effect.
"More studies of other cranberry products such as tablets or capsules may be justified, but only for women with recurrent UTIs, and only if these products contain the recommended amount of active ingredient," said Dr Jepson.