Friend, 30mg of Zyrtec is Not going to be fatal for a 45 lbs Reese. However, you may see some vomiting, sedation, etc. otherwise he will be fine. This is what you can do:
1- Do not give things like Pepto Bismol or imodium. Start him on Pepcid AC (famotidine
) 10mg 1/2 tablet every 12 hours for the next 2-3 days. You can buy Pepcid AC over the counter in any pharmacy.
2- No food or water for 8-10 hours. Need to give his gastrointestinal tract time to rest.
3- After 8-10 hours offer a small amount of water. Wait half hour, if no vomiting, offer small amount of rice (50%) and boiled boneless chicken or ground beef (50%) 4-6 times a day. Keep on this diet for several days then wean into regular diet.
4- If he continues to vomit or lethargic after doing the above he will need to be seen by your family veterinarian for a hands on examination.
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