Effectiveness of cranberry capsules to prevent urinary tract infections in vulnerable older persons: A double-blind randomized placebo-controlled trial in long-term care facilities 

In a double-blind, randomized, placebo-controlled trial from 2014, 928 seniors (703 were female) from long-term care facilities were divided into low-risk and high-risk groups for UTIs. Participants were given either 500 mg cranberry extract or placebo twice daily for 12 months. Those defined as “high-risk”, which included participants with long-term catheterization, diabetes or at least one UTI in the previous year, at baseline had a lower incidence of clinically diagnosed UTI. Positive effects were seen from two months of follow-up onward. 

References:

Calijouw MAA, van den Hout WB, Putter H, et al. (2014). Effectiveness of cranberry capsules to prevent urinary tract infections in vulnerable older persons: a double-blind randomized placebo-controlled trial in long-term care facilities. J Am Geriatr Soc. 62(1): 103-10