If you know your dog has cancer then obviously you've already taken him or her to the vet, right? Well, what would you want if you had cancer? What stage is it in? I would spend as much quality time with that dog as possible and make it feel loved. If you have to end the pain then do it RIGHT! Most people will disagree with that but most people think about themselves instead of the dog. F that!
Take him to the veterinarian. See what they tell you. Maybe they can help the dog.
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