In most cases you should not be too concerned about your dog vomiting. If you know that the dog got into something or you changed its diet, you can probably wait 24 hours to see if the vomiting clears up, but only if the dog does not appear lethargic. You must withhold food for 24 hours and then gradually start a bland diet. If at any time the dog becomes worse please see your veterinarian If your dog has been vomiting continually for more than 1 hour or if he as been vomiting on and off for.