Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about your wee one’s situation, and wanted to help.
Based on the dose he has had, this would be a 2.5x overdose for your lad. Therefore, we could see adverse effects like GI upset (vomiting, diarrhea, appetite loss) and while the kidney effects from this drug isn't as likely at this dose, we do have the risk of a stomach ulcer/perforation. Therefore, we do want to be proactive and keep a close eye on him.
Since it has been just under 2 hours, we can still induce vomiting now. 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 apomorhpine (a very strong injectable emetic) can be administered just get this out of the stomach and avoid any adverse issues.
As well or alternatively, 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, since this drug can cause GI issues, 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, we can also treat with an antacid. And this is advisable for protecting that stomach. There are a number of antacids that are available over the counter and pet friendly. I would advise only treating with one, but the two I tend to use are: 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). These are usually given 20 minutes before offering food (to allow absorption) and of course you want to double check with your vet before use if your wee one has any pre-existing health issues or is on any medications you haven't mentioned.
Overall, this is a large dose and GI signs are a risk here. Therefore, we'd want to use the above at this stage. Otherwise, we need to monitor for the above over the next few days. Though if you see any belly pain, paling gums bloody/brown vomit or black stool (though be aware that the charcoal will cause this too and that isn't a worry) then we'd need him seen urgently for a possible stomach ulcer that will need further treatment to heal. But if we act fast, we can hopefully avoid any harm here.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,
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