Friend, vomiting can be caused by many different medical conditions as you may already know. It will be difficult for me to determine the exact underlying cause without a hands-on examination and some testing. But, common causes are a simple gastritis, dietary indiscretion, intestinal parasites, ingestion of a foreign body, infections, etc. to name a few causes. In this case, the fact that she ate soup with onions I would place this very high on the list of possible causes. You see, onions are toxic to dogs; they can cause vomiting, diarrhea, damage to red blood cells leading to anemia, etc. So, if you are seeing evidence of the soup in the vomit this in a way is good because it decreases the toxic potential. In response to your question, do not give Zofran, instead, this is what you can do:
1- Do not give things like Pepto Bismol, Imodium or Zofran. Start her on Pepcid AC (famotidine) 10mg 1/2 tablet every 12 hours for the next 2-3 days. You can buy Pepcid AC over the counter in any pharmacy.
2- No food or water for 8-12 hours. Need to give her gastrointestinal tract time to rest. Keep her indoors in a cool room when fasting.
3- After 8-12 hours offer a small amount of water. Wait half hour, if no vomiting, offer a small amount of rice (50%) and boiled boneless chicken or ground beef (50%) 4-6 times a day. Keep on this diet for several days then wean into regular diet.
4- If she continues to vomit after doing the above she will need to be seen by your family veterinarian for a hands on examination. If she develops pale gums, becomes lethargic, develops bloody stool, abdominal swelling, etc. have her checked by ER veterinarian.
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