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This amount of milk chocolate is unlikely to cause a normal size Lab much of an issue. She might have some diarrhea but it is not likely to be toxic. However, I will give you the information on chocolate toxicity.
First thing is to not panic. Different kinds of chocolates have different toxicity amounts. Chocolate contains theobromine, which is what causes the toxic response. If the dog ate the chocolate recently (within last 2 hours), you can induce vomiting with 1 teaspoon of hydrogen peroxide 3% per 10 pounds, which can be repeated 10 minutes later if it does not induce vomiting. Items such as cookies and cakes will have less actual chocolate in them, so you will have to make an estimate in the amount of chocolate that may have been in the amount of cake, brownie, or cookie your dog may have ingested .
Signs of theobromine toxic poisoning should appear within 1 to 4 hours of ingesting the chocolate. These include vomiting, increased thirst, diarrhea, weakness, difficulty keeping balance, hyperexcitability, muscle spasm, seizures, coma and potentially death from an abnormal heart rhythm.
The following websites will give you’re the approximate amounts of chocolate that you will need to worry about based on the size of your dog and the type of chocolate. Consult this page to determine if a toxic amount has been ingested.
If the dog had ingested enough to be toxic according to the chart, you would need to have the dog taken to the vet for supportive care. The vet can administer charcoal to absorb the toxins and let it pass through the dog’s system with less absorption and monitor the dog’s vital signs until the danger has passed.
You also should be aware of the fact that the high fat content in chocolate can also trigger pancreatitis in some dogs and she may wish to take your dog into the vet to avoid this condition developing and definitely keep an eye out for symptoms of this.. Still this is probably not an issue in this particular case. You can read about pancreatitis here:
Hope this helps.