The apple cider vinegar can be added to the dog's water or put directly on his food. The natural type from a health food store, not the pasteurized version from the supermarketTwo tablespoons of tomato juice on the dog's food twice a day will have the same effect. . Insect Stings to Dogs or Cats Slice a raw onion and apply to the sting as soon as possible.If the sting is inside the mouth - Keep "Antihistamine" capsules in your first aid kit. A rapid administration soon after the sting can prevent serious complications.