Panacur is a very safe dewormer, it can cause vomiting and diarrhea. But, the fact that she is lethargic I am more inclined to believe this is related to the Pizza. You see, onions are toxic to dogs. In addition, the fat content and condiments in Pizza can also cause vomiting, diarrhea, pancreatitis, etc. I am more concern about the onions and the possibility of pancreatitis.
Is dehydration a concern at this point? In any case of vomiting we are always concern about dehydration. But, at this stage in such a short period I doubt she is in a life-threatening state of dehydration. The difficulty that we face now is that if she is vomiting it will be difficult to maintain her hydrated by giving her water orally. The recommendation at this point is to not give anything orally so that when you do offer water she keeps it down.
In this case, unfortunately, we are somewhat limited as to what you can do at home. I would strongly recommend having her checked this morning by your family veterinarian for supportive care (IV fluids), blood work to rule out pancreatitis and monitor her red blood cells as a suspect of onion toxicity. In the meantime, this is what you can do:
1- Do not give things like Pepto Bismol or Imodium. Start her on Pepcid AC (famotidine) 10mg tablet every 12 hours for the next 2-3 days. You can buy Pepcid AC over the counter in any pharmacy.
2- If your local vet opens in 3 hours, keep her off food and water until she is seen by your veterinarian. If you feed her or give her water she may vomit, not only will she not absorb the water but also loose electrolytes in the vomit making matters worst. So, right now all you wanna do is stop her from vomiting. Do Not let her eat grass either.
3- Follow up with your family veterinarian for a hands on examination and supportive care.
That is good she had the energy to go out. I would try to keep her confined in a cool room when fasting.
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