A study in the United Sates has concluded that whilst cranberry capsules can reduce the prevalence of urinary tract infection (UTI), cranberry juice is less effective. The study of 160 patients showed a reduction of risk of UTIs by 50% when using cranberry capsules.
Cranberry juice is seen as less effective due to the large amounts that need to be consumed to match the levels concentrated in capsules. The concentrated levels in cranberry capsules contain A-type proanthocyanidins (PACs), which interfere with the bacteria's ability to the bladder wall thereby reducing the likelihood of infection.
You can read more about this on the Medical News Today website.