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Cranberry to prevent UTI


Bandolier first looked at the efficacy of cranberry juice in preventing urinary tract infections in issue 6, a little while ago. Subsequent research proved to be sporadic and complicated, but a sensible clinical picture now appears to be emerging, so it seems a useful time for an update, and a quick Bandolier review.

Cranberry juice in various forms has been tested for various different effects in different situations. What we can say, with reasonable certainty, is that cranberry juice does not affect urinary tract infection rates in people with neurogenic bladder; a number of randomised trials all come to the same conclusion. Nor does cranberry juice seem to affect bacterial colonisation or infectious diseases generally in children.

The one area where there appears to be consistent benefit is in preventing urinary tract infection in susceptible women.


Bandolier sought randomised trials of cranberry juice, or extract, in whatever form and in whatever combination with other products, in studies looking at prevention of urinary tract infection. We looked in various electronic databases, and examined bibliographies of relevant papers.


There have been four trials [1-4], with 779 participants, 85% of whom were women. Brief details of the trials show similarities and differences:

Trial 1 was conducted in elderly women (mean age 79 years) over six months, comparing 300 mL cranberry juice cocktail with a cocktail not containing cranberry juice.

Trial 2 was conducted in young women (mean age 42 years) who had at least two urinary tract infections in the previous year. It compared cranberry juice and cranberry extract in tablets against placebo over 12 months.

Trial 3 was also conducted in young women (mean age 30 years) who had at least one E coli urinary tract infection. It compared a cranberry juice cocktail against placebo and lactobacillus drink over six months.

Trial 4 was conducted in elderly patients (mean age 81 years, two thirds women) in hospital, and compared cranberry juice against placebo for less than a month.

All reported an outcome of patients with urinary tract infection, usually defined as clinical symptoms confirmed by culture (usually 100,000 colony forming units per mL).


Combining results for urinary tract infections from all four trials, there were 42/409 (10%) in patients randomised to some form of cranberry product, and 64/370 (17%) in those randomised to a non-cranberry placebo. The relative risk was 0.5 (0.4 to 0.8), and the number needed to treat to prevent one urinary tract infection was 14 (8-45).

Individual results for the four trials are shown in Figure 1, where the dark symbol represents the shortest trial of less than one month [4]. The other studies lasted six or 12 months, and the earliest [1] also showed no difference after only one month, but consistent difference thereafter (see Bandolier 6).

Figure 1: Effect on urinary tract infection rates with cranberry product and placebo

Analysing for only the longer duration studies, there were 35/222 UTIs (16%) in patients randomised to some form of cranberry product, and 50/181 (28%) in those randomised to a non-cranberry placebo. The relative risk was 0.5 (0.4 to 0.8), and the number needed to treat to prevent one urinary tract infection was 8 (5-27).


What is interesting is the consistency of effect between these four trials, conducted in different patients, in different countries, over different periods, and using different cranberry products, although all the patients were at increased risk of urinary tract infection. In each the number of urinary tract infections was reduced by about half. In practical terms, it means that for every 10 women at increased risk of urinary tract infection who take cranberry juice daily for six or 12 months, one fewer will have a urinary tract infection.

Two further thoughts. One is about issues of validity in clinical trials, and two [1,4] have now shown less impressive effects over the short term. Any future trials should reflect the likely longer-term use, though it was entirely reasonable to conduct a short term study in hospital patients [4], in whom a long stay was unlikely.

The second is how we can express these results to women who might want to consider using cranberry juice themselves. Perhaps we should say that over six months to a year of cranberry juice use:

Maybe this expresses both the fact that it works, but that it is no miracle cure, and won't work for everyone, always.


  1. J Avorn et al. Reduction of bacteriuria and pyuria after ingestion of cranberry juice. JAMA 1994 271: 751-754.
  2. T Kontiokari et al. Randomised trial of cranberry-lingonberry juice and Lactobacillus GG drink for the prevention of urinary tract infections in women. BMJ 2001 322: 1571-1573.
  3. L Strothers. A randomized trial to evaluate effectiveness and cost effectiveness of naturopathic cranberry products as prophylaxis against urinary tract infection in women. Canadian Journal of Urology 2002 9: 1558-1562.
  4. ME McMurdo et al. Does ingestion of cranberry juice reduce symptomatic urinary tract infections in older people in hospital? A double-blind, placebo-controlled trial. Age and Ageing 2005 34: 256-261.

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