Now since we cannot predict how much chocolate/cocoa powder was in these muffins, we are best to intervene. Especially since this dose is likely to cause GI upset (vomiting, diarrhea) even if it doesn't cause neurological signs.
Now while we cannot be sure how long ago Denver had these, I would suggest trying to induce vomiting just on the chance some of this is still in the stomach (and thus can be purged)To induce vomiting at home, you can administer 3% hydrogen peroxide orally at a dose of 1ml per pound. (2 teaspoons per 10 pounds of body weight). You can give it via dropper, syringe, turkey baster – we just want to get it in. After giving this orally, move the abdomen around or get your wee one walking about to get things mixing. This should usually lead to vomiting. If it is unsuccessful after 10 minutes then it can be repeated twice more. And if we still have no vomiting, then you'd need to consider seeing your local vet (or ER vet) so that apomorphine (a very strong injectable emetic) can be administered just get this out of the stomach and avoid any adverse issues.
As well or alternatively at this point, you can also consider administering activated charcoal at this stage. This is available over the counter from the pharmacy (ask for the high strength version, not the one for gas) and works by binding any remaining material in the stomach. For activated charcoal, we tend to give 1-4 grams per pound every 8 hrs. This can be mixed with food to be fed or with water to syringe feed (do note that it stains, so keep it away from white carpets/clothes). This will just limit how much is absorbed and reduce the intoxication risk here.
Finally, after the above, we do want to try to address any potential GI upset here for the next 24-48 hours. To do so, you can consider offering a light diet option for a few days. Examples of an easily digestible diet include cooked white rice with boiled chicken, boiled white fish, scrambled egg, or meat baby food (as long as its free from garlic or onion powder). Ideally, we want to offer this as small frequent meals to keep the stomach settled.
Further to this, if we see any nausea, we can also treat with an antacid like Pepcid (More Info/Dose @http://www.petplace.com/article/drug-library/library/over-the-counter/famotidine-pepcid) or Zantac (More Info/Dose @ http://www.petplace.com/article/drug-library/library/over-the-counter/ranitidine-hcl-zantac). Whichever you use, we'd give this 20 minutes before food to allow absorption and of course double check with your vet before use if your wee one has any known health issues or is on any medications you haven't mentioned.
Overall, I am concerned about how much cocoa/chocolate was in these muffins. Often we don't see adverse signs until 4-36 hours later .So, I'd lean to the above for him to offset any risk. Else we'd need to monitor and use the above supportive care for his stomach over the next 48 hours if any signs were to arise from these,
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