Please call the ASPCA Poison Control Hotline
ASPCA | Animal Poison Control Center their phone number is (888) 426-4435. There is a $65 fee but they will be able to tell you based on the weight of your pet and what they have ingested what is the best course of action. They will give you a case number which you can provide to you vet/emergency vet and they can speak with with for free for as many consultation that you need regarding this specific case. The ER Vets usually end of calling the hotline anyways and passing the charge on to you as part of the emergency visit. I hope your dog is feeling better soon. There is a great sticky in the YT library for chocolate.
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