Friend, your dog based on weight took a dose of 0.6mg/kg of Methimazole; at this dose, the worst you can expect is mild gastrointestinal symptoms like vomiting or diarrhea, otherwise, she will be fine. I would keep an eye on her for the next 24 hours, if she develops vomiting or diarrhea this is what you can do:
1- Do not give things like Pepto Bismol or Imodium. Start her on Pepcid AC (famotidine) 10mg 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 6-8 hours. Need to give her gastrointestinal tract time to rest. Keep her indoors in a cool room when fasting.
3- After 6-8 hours offer a small amount of water. Wait half hour, if no vomiting, offer a 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 she continues to vomit after doing the above she will need to be seen by your family veterinarian for a hands on examination.
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