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Tagamet (cimetidine) is frequently used in dogs. Dose is 3 - 5 mg per pound of body Weight. So, for him, one 300mg tablet once or twice a day would be an appropriate dose. If his symptoms don't improve in just a few days, he needs to see a veterinarian. Don't let him eat grass, it will only make things worse. Many people believe dogs eat grass because they have an upset stomach. Not true. They eat grass because they like it, but it is terribly irritating to the stomach and small intestine. That, actually may be the cause of his discomfort.

Doctor W
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No doctor patient relationship is formed by this communication. This advice is no substitute for a complete physical examination and appropriate diagnostics provided by a licensed veterinarian.