According to the ASPCA, as of June 2014 there is no current indication that cranberries are bad for dogs. However, it is important to speak to a veterinarian before giving a dog cranberries or cranberry juice for a urinary tract infection or any other reason.
Although there is no evidence that cranberries are toxic to dogs, they can cause gastrointestinal upset. This is because dogs have a tough time digesting plant material.
If the purpose of giving the dog cranberries is to get rid of a urinary tract infection, it is crucial to look at the underlying issue. If a dog is getting these infections regularly, take the animal to a vet to determine the best course of action.
There actually is no clear evidence of cranberries or cranberry juice being an effective treatment for urinary tract infections in dogs. Since it is a common remedy for humans, people have got the idea that it is a good treatment for dogs and other pets.