Research at a Scottish university has questioned the effectiveness of cranberry juice in protecting women against a painful bladder infection.
Cranberry juice is said to alleviate cystitis, a stinging inflammation of the bladder usually caused by a urinary tract infection (UTI).
The University of Stirling reviewed 24 studies involving 4,473 participants.
It said the new analysis suggested cranberry juice provided "no meaningful protection" against cystitis.
The research has been published in the Cochrane Library, which specialises in assessing medical evidence to inform guidelines and health policy.
The university said it was believed that compounds in cranberries may stop bacteria sticking to cells lining the walls of the urinary tract.
Some of the studies reviewed showed "small benefits" for women suffering from recurrent infections.
However, the studies' 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.
Dr Ruth Jepson, lead researcher for the Stirling review, said cranberry juice was less effective than previously thought.
She added: "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."
However, Dr Jepson said more studies may be needed to better understand the effectiveness of cranberry tablets and capsules for women with recurrent UTIs.