Friend, 3.75 oz of milk chocolate will not be life-threatening or systemically toxic to a 78 lbs dog, he may develop mild vomiting or diarrhea otherwise he will be fine. No need to make him vomit with peroxide. I would keep an eye on him if he vomits or develops diarrhea this is what you can do:
1- Do not give things like Pepto Bismol or Imodium.
2- No food or water for 6-8 hours. Need to give his gastrointestinal tract time to rest.
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 he continues to vomit after doing the above he will need to be seen by your family veterinarian for a hands on examination. Symptoms that would require an ER vet visit are abdominal swelling, pale gums, hyperactivity or profound lethargy, muscle tremors, etc.
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