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Hello. Welcome to JustAnswer. I am Dr Z. I'm reviewing your question now, and will post back with a reply ASAP.
Hello and thanks for trusting me to help you and your pet today. I am a licensed veterinarian with over 30 years of experience and would be happy work with you.
Since this occurred last night, inducing vomiting would have no benefit. Trifexis is a combination of milbemycin and spinosad. The milbemycin os of no consequence, but the spinosad, in higer doses can be a problem. Fortunately, we don't usually run into problems until an ingestion of over 400 mg/kg of spinosad. It looks like the tablet he ingested contains around 1620 mg and for a ten pound dog, that would be a dose of approximately 360 mg/kg.
With this dose, we would more likely only see GI irritation, but if he were particularly sensitive, we could see neurologic symptoms as well, such as tremors or even seizures. The latter signs however, would more than likely have shown up already and not show up over 12 hours later. His not eating today however may be a result of GI irritation still.
Since he is an adult dog, a 12-24 hr fast will not hurt him and will let his gut calm down. Continue to offer small amounts of water at a time. When you do begin feeding him again, you will want to offer frequent, small meals of something very digestible for him. These would include items like boiled hamburger or chicken and rice or pasta. No sauces or gravies, but a little non greasy broth would be OK. If he starts to vomit or won't eat after fasting, I would definitely seek veterinary intervention. It would also be OK to administer the over the counter antacid called Pepcid AC (famotidine). Give one 20 mg tab twice a day for the next 24 hours.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.
My posted replies are for general education only and not meant as a diagnosis. Only after a thorough veterinary examination can a diagnosis for your pet be made and specific treatments be advised or medications be prescribed.
Yes, they will sometimes even shake if they have an upset GI system.
If you are still concerned, you may want to seek ER services for at least an exam.