Cranberry juice

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Hi, A friend of my gives her dog cranberry juice, she said it helps with preventing bladder infections. We got into a big agruement because I said I didn't think it was good for her boxer to have cranberry juice, she said it was ok because she read it off the internet, which I kindly reminder that not everything on the internet is true. Could someone please tell me if its ok or no ok for dogs in general to have cranberry juice.

Thanks