I have written previously about the rationale and evidence concerning the use of cranberry products to prevent or treat urinary tract infections. In my summary, I concluded:
There is weak theoretical justification for using cranberry products for urinary tract infections (UTI), though none of the supporting preclinical evidence involves dogs or cats. There is conflicting clinical trial evidence in humans, and no clinical studies in dogs and cats.
A recent Cochrane review concerning the use of cranberry products to prevent UTIs in humans further undermines the already weak argument for the value of these products. This update of the previous review in 2008 includes an additional 14 clinical trials in a meta-analysis. The conclusion was that there is no evidence cranberry products are effective for preventing UTIs:
Data included in the meta-analyses showed that, compared with placebo, water or not [sic] treatment, cranberry products did not significantly reduce the occurrence of symptomatic UTI overall (RR 0.86, 95% CI 0.71 to 1.04) or for any the subgroups: women with recurrent UTIs (RR 0.74, 95% CI 0.42 to 1.31); older people (RR 0.75, 95% CI 0.39 to 1.44); pregnant women (RR 1.04, 95% CI 0.97 to 1.17); children with recurrent UTI (RR 0.48, 95% CI 0.19 to 1.22); cancer patients (RR 1.15 95% CI 0.75 to 1.77); or people with neuropathic bladder or spinal injury (RR 0.95, 95% CI: 0.75 to 1.20).
Prior to the current update it appeared there was some evidence that cranberry juice may decrease the number of symptomatic UTIs over a 12 month period, particularly for women with recurrent UTIs. The addition of 14 further studies suggests that cranberry juice is less effective than previously indicated….Given the large number of dropouts/withdrawals from studies (mainly attributed to the acceptability of consuming cranberry products particularly juice, over long periods), and the evidence that the benefit for preventing UTI is small, cranberry juice cannot currently be recommended for the prevention of UTIs. Other preparations (such as powders) need to be quantified using standardised [sic] methods to ensure the potency, and contain enough of the ‘active’ ingredient, before being evaluated in clinical studies or recommended for use.
Cranberry juice does not appear to have a significant benefit in preventing UTIs and may be unacceptable to consume in the long term. Cranberry products (such as tablets or capsules) were also ineffective (although had the same effect as taking antibiotics), possibly due to lack of potency of the ‘active ingredient’.
This review indicates pretty clearly that overall, cranberry juice is not effective in preventing UTIs despite theoretical reasons why it might be. This illustrates, yet again, why we cannot rely on extrapolation from pre-clinical or in vitro studies to tell us what will work in actual patients.
As always, one cannot completely rule out a subset of patients for whom these products might have benefits, but quite a variety of patient populations and different forms of cranberry have been investigated so far without any convincing evidence of value. And, of course, there is no clinical research at all in veterinary species to support using cranberry products to treat or prevent UTIs.