Cranberry seems to work because a proanthocyanidins (an antioxidant) prevents the E. Coli from attaching to the cell walls in the bladder. In vitro studies have confirmed that proanthocyanins do cause deformation of the E. coli cells and thus suggests prevents them from adhering to the bladder walls. The adhesion in one study was reduced by 75%
There are no studies in cats.
There is a study in beagles showing that the metabolites of the cranberry do appear to make it to the bladder and do reduce the adhesion of the E. coli. The researchers administered 1 Crananidin tablet per day and the dose peaked at 7 days after which it remained steady.
There are human studies that show no significant improvement with the use of cranberries. Several of the studies do comment that perhaps the potency of the cranberry used in the study was not sufficient.
So what does this all mean? It does seem that cranberry can prevent UTIs in cell cultures, but there is no evidence that it does in cats.
There are however many anecdotal accounts by cat owners that cranberry pills, cranberry powder, and even cranberry juice has helped cats. I know from personal expereience, I have used cranberry powder with cats exhibiting UTI symptoms and the symptoms disappeared in 2-3 days. Was it coincidence? Maybe, maybe not. I do know that 1/2 a cranberry extract pill is a much larger dose for a cat than 1 pill 2x a day is for a person. So maybe, the proanthocyanins did reach a therapeutic level in my cats.
There are also a couple of concerns about using cranberries with cats. One is cranberries contain salicylic acid (aspirin). Now, before people panic, cranberry juice contains 7mg/L of juice. Presumably poders and extracts would contain more. But.... for cats, the toxic level for aspirin begins at about 50mg/kg of body weight. So an 11 lb cat (5 kgs) would need 250mg to reach a minimum toxic level, of about 37 quarts (35L) of juice.
The other concern is the presence of oxalates. Oxalates have been tied to the formation of oxalate kidney stones in humans, but cats rarely develop kidney stones -- they are more likely to have bladder stones, so I am unclear if this is even a concern. In any case, cranberries contain 5-7 mg of oxalates per 3.5oz of food. This is the same level for pumpkin, which is wildly used for cats and regarded as beneficial.
Bottomline, if I have another cat with UTI symptoms, I will use it again. If and when the researchers ever do a sound study on felines and show it does not work, then I may rethink my position.
Summary of scientific studies from "Complementary and Integrative Therapies" by Donna Raditic, in Vet. Clinics of N. Am.: Small Animal Practice, July 2015