Can you give a dog zantac?

If a vet prescribes it and only exactly as they say. You would need to ask a vet before giving anything. Something like that would need to be carefully considered and interacts and has an effect on other medications that your pup would be likely to be on if a med like that is indicated and would benefit your pup. It may just cause a problem and toxicity is common. The only person that would know enough about the meds, the pup and the situation as a whole to be able to prescribe and give dosage if it was okay would be the veterinarian handling the treatment.
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