Cranberry juice provides no meaningful protection against cystitis, contrary to the belief of many women.
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, say scientists.
Cystitis is a stinging inflammation of the bladder usually caused by a urinary tract infection (UTI).
Numerous websites and many GPs encourage women troubled by the infection 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 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 did show "small benefits". 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.