Cranberry juice is unlikely to prevent bladder and kidney infections, according to an updated systematic review published in The Cochrane Library. The authors analysed the most up-to-date evidence and concluded that any benefit, if present at all, is likely to be small and only for women with recurrent UTI.

Urinary tract infections (UTIs) affect the bladder, as in cystitis, and sometimes the kidneys. Cranberries and cranberry juice have been used to prevent UTIs for decades, although it is not clear how they might help protect against infection. According to one theory, certain sugars and flavanol compounds in cranberries prevent bacteria sticking to cells lining the walls of the urinary tract. Several systematic reviews have been published on the subject in The Cochrane Library, each time incorporating more evidence. In the last review in 2008, it was concluded that cranberries offer a small benefit in preventing recurring UTIs in women.

In the current review, the researchers gathered together evidence from 24 studies that involved a total of 4,473 people. These studies included 14 added since the 2008 update. Those in treatment groups were given cranberry juice, tablets or capsules, while those in control groups were given placebo cranberry products, water, methenamine hippurate, antibiotics, lactobacillus or nothing. Although in some studies there were small benefits for women suffering from recurring infections, women would have to consume two glasses of cranberry juice per day for long periods to prevent one infection. The researchers conclude that current evidence does not support cranberry juice as a means of preventing UTIs.

"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," said lead researcher Ruth Jepson of the University of Stirling in Stirling, UK. In the studies where participants were given juice, there were large numbers of drop-outs, suggesting it might not be acceptable to drink over long time periods. A common problem with the studies evaluating cranberry tablets or capsules was that they rarely reported the amount of active ingredient, so it was unclear whether levels would have been high enough to have any effect.

"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," said Jepson. "More studies of other cranberry products such as tablets and capsules may be justified, but only for women with recurrent UTIs, and only if these products contain the recommended amount of active ingredient."

Funding for the 2012 update of the review was provided by the UK NHS NIHR.