Research has shown that drinking cranberry juice makes the urine more acidic. Since common urinary tract infections in pets (especially dogs) are caused by bacteria such as E. coli, which function best in alkaline urine, many holistic doctors promote cranberry juice extracts for treating bladder infections. Additionally, since the most common bladder stones in dogs and cats,and the sand-like gravel and microscopic crystals that are often encountered in cats with feline lower urinary tract disease (FLUTD, formerly called FUS form in alkaline urine, acidifying the urine with supplements such as cranberry extracts may prove helpful.
However, contrary to early research in people, it now appears that acidification of the urine is not so important as cranberry's ability to block bacteria from adhering to the bladder wall. Preventing bacterial adhesion to the bladder wall prevents infection and allows the bacteria to be washed out with the urine.
Cranberry juice is believed to be most effective as a form of prevention. When taken regularly, it appears to reduce the frequency of recurrent bladder infections in women prone to develop them. Cranberry juice may also be helpful during a bladder infection but not as reliably. Similar findings are lacking in pets but may be applicable.
In people, the recommended dosage of dry cranberry juice extract is 300 to 400 mg twice daily, or 8 to 16 ounces of juice daily. Pure cranberry juice (not sugary cranberry juice cocktail with its low percentage of cranberry) should be used for best effect.
For pets, the recommended dosage varies with the product. One recommended product used in cats recommends a daily dose of 250 mg of cranberry extract. Cranberry juice is not recommended as it is all but impossible to get most pets to drink enough of the juice to be effective.
There are no known risks of this food for adults, children, or pregnant or nursing women, nor are there any known risks in pets. However, cranberry juice may allow the kidneys to excrete certain drugs more rapidly, thereby reducing their effectiveness. All weakly alkaline drugs may be affected, including many antidepressants and prescription painkillers.
In dogs and cats, the push to acidify the urine through prescription-type diets has led to a slight increase in oxalate stones, which are more common in acid urine. Pets taking cranberry extract would be more prone to develop crystals and stones, such as oxalate stones, which are more common in acid urine. However, since the crystals and stones that form in alkaline urine are much more commonly diagnosed, pets with chronic stones (and cats with chronic FLUTD) would probably benefit from acidification of the urine even with the slight risk of stones forming in acid urine. Discuss this with your veterinarian.