You've probably heard that drinking cranberry juice is good for preventing urinary tract infections (UTIs). There is some evidence that it's true, but it ain't easy.
The strongest evidence available for prevention is among adult women with previous UTIs. In this group, results repeatedly show about a 50% reduction in disease recurrence with regular juice consumption. In elderly and pediatric patients, and some patients with bladder problems, the effect is less pronounced.
A caveat made in many of the prevention studies is that the cranberry juice’s effects seem to be dose-dependent, and that the amount a person would need to drink daily to have a real preventative effect is more than what most people are willing to drink. In some studies, as many as half of the participants withdrew before the studies’ completion, suggesting that prolonged, regular guzzling of cranberry juice is too much to bear, either because of stomach aches, the calorie load, or adverse interactions between the juice and certain medications.
Or maybe it's just because many people think cranberry juice is vile. Anyway, scientists aren't quite sure why the juice has an effect on UTIs, but there are several plausible theories, which you can read about at mental_floss. Link
(Image credit: Flickr user Sally Crossthwaite)