Short answer: call your vet or a pet poison line now, even if your dog looks fine. Do not wait for signs.
What to gather first
The ASPCA says to collect any remaining packaging so the vet can tell what type of chocolate it was and how much your dog ate. Note any fillings, such as raisins or macadamia nuts, because they can also be toxic. If the chocolate is sugar-free, it may contain xylitol, which is a separate danger. Know your dog's weight. For more on this, see Which foods are unsafe for dogs, and what should I do if mine ate one.
Why it matters
Chocolate contains theobromine, which dogs are sensitive to. In large amounts it can cause hyperactivity, a fast heart rate, muscle tremors, and seizures. Generally, the darker the chocolate, the more theobromine it holds. It takes much less chocolate to be a problem for a small dog than a large one, so take any amount seriously.
Who to call
Your vet or the poison line will use the type, amount, your dog's weight, and medical history to tell you what to do. That may be to watch at home, or to come in for treatment. If your dog has mild signs such as vomiting, diarrhea, extra thirst, or restlessness, still call.
What not to do
Do not try to make your dog vomit unless a vet tells you to. Do not wait to see if signs appear.
Prevention
Keep chocolate, cocoa, baked goods, and bags and purses out of reach, especially around holidays. Tell guests to keep their things off the floor.
This article is general education, not veterinary advice. If your dog seems unwell, is in pain, or something changed suddenly, call your vet. If it could be an emergency, do not wait.
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Frequently asked questions
What do I do right now?
Call your vet or a pet poison line, even if your dog looks fine. Do not wait for signs to appear.
What do I gather first?
Any remaining packaging so the vet can tell what type of chocolate it was and how much was eaten, plus any fillings such as raisins or nuts.
Why is chocolate dangerous?
Chocolate is unsafe for dogs, and darker chocolate is riskier. Your vet will judge what the amount means for your dog.
What will the vet ask?
The type, the amount, the weight of your dog and their history, then advise watching at home or coming in.
