Welcome to Justanswer! I am Dr. Altman and happy to assist you both today!
I am glad to hear that she vomited up a lot of what she ingested, very likely the amount of onions was not enough to cause issues as the onion powder tends to be the most toxic form of onions in pets.
Since there is continued actively vomiting we want to minimize anything orally to stop the nausea and abdominal cramping resulting in vomiting. I know the thought is she needs to drink to maintain hydration but if she is actively vomiting even water she is becoming more dehydrated with water intake.
If there is no improvement then please consider having your dog seen by a veterinarian sooner rather than later. They can give injectable medications to bypass the intestinal tract and hopefully get the nausea under control
1- Keep indoors to avoid eating grass/ leaves until morning
2- Offer a small amount of water (1-2 teaspoons hourly) for a few hours tomorrow morning
3- Start Pepcid (famotidine) 0.25 mg/ pound every 12 hours OR prilosec (omeprazole) 0.5 mg/ pound daily for the next 3 days with the second administration of water (crushed if needed)
4-If no vomiting, offer small amount of white rice and boiled boneless or ground chicken or ground beef. For example, one tablespoon hourly for a few hours then slowly start to increase the amount while reducing the frequency. Keep on this diet for several days then wean into regular diet.
5- If vomiting continues after doing the above, I would encourage having your family veterinarian perform an examination to determine the next step to getting Millie comfortable.
Indications your dog should be seen by a veterinary er sooner include: sticky gums that are pale, yellow, blue, purple, temperature rectally >103.5, progressive lethargy with less responsiveness to you, vomiting or diarrhea that includes blood, resting respiratory rate (breaths per minute) >60, swollen abdomen.
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