Can we distinguish truth from fiction when we dose our bladders with fruit? Cranberry juice has been popular for treating urinary infections for decades and it's hard to think of a nicer and more natural therapy. Which, unless we choose to believe that whatever sounds pleasant must automatically be true, doesn't tell us a thing worth knowing.

It's easy enough to make up stories about why cranberry juice should work. Cranberries, for example, contain molecules that seem to stop bacteria sticking to the bladder wall. That should stop infections. But lots of therapies that seem perfectly reasonable have turned out to be useless or even harmful. Trying to figure out if there are reasons why cranberry juice should work is one thing: finding out if it does is actually something completely different. And for that we don't need expert opinions, whether they belong to cranberry farmers or self-proclaimed health food experts. We need reliable trial data.

For a change, we actually have it. In two high-quality randomised placebo-controlled trials, women who drank cranberry juice or took cranberry tablets had fewer urinary infections than women who didn't. Those who took cranberry juice were 39% less likely to develop infection in the following year than women who didn't.

Cranberry juice prevents urinary infections: a few of them anyway. We have no good evidence, unfortunately, about whether it's any use for treating an infection that has really got its teeth into you. There are plenty of theories and masses of wholly unreliable data, but nothing - unfortunately - to justify turning the flat fenland of Norfolk into a colourful cranberry swamp.