Can you describe exactly what symptoms your dog is showing? Is he actually vomiting? Does he have loud abdominal noises and gas? Any diarrhea or constipation? Has this happened before and how old is your dog? If he is not eating or drinking, how long has it been? Is he lethargic or basically keeping his usual activity level?
I have a Greyhound also. As Jaybay asked, need more symptoms.
If it is an upset stomach (which mine gets on and off) you can give 1 Pepcid to see if that settles the stomach. My Grey went through this just yesterday, as a matter of fact and he didn't eat all day but was drinking, which is important. I put down half his usual food ration this morning and added some boiled chicken and brown rice. He at it and his stomach seems fine now.
If you dog has bad diarrhea and/or vomiting, then I would be calling the Vet.
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