So, the good news is that even in massive overdoses like this (roughly 1000mg/kg of calcium carbonate) we are not likely to see serious side effects, unless the dog has previous kidney disease or other medical problems. I would definitely expect vomiting and diarrhea from this, but it should not last more than 24 hours or so. Usually, this side effect would start within 6-12 hours of ingestion of the Tums.
When this has JUST happened (last 2 hours) I usually do recommend that people induce the dog to vomit, as that will be less vomiting than I would expect if the Tums stayed in.
To make my own dog vomit at home, I give 3% hydrogen peroxide. The dose is 2 teaspoons per 10lbs body weight (which works out to about 1/3 cup in a dog of this size).
You can repeat in 10 minutes if he has not vomited yet.
More here: http://www.petplace.com/dogs/how-to-induce-vomiting-emesis-in-dogs/page1.aspx
Also, some hints to make it work better:
1. Feed a small meal or slice of bread first - they are more likely to vomit with some food in the stomach.
2. Dogs will often drink peroxide if you mix it 50:50 with milk or ice cream, and it is just as effective.
3. Dogs are more likely to vomit after getting hydrogen peroxide if they move around - play ball, go for a walk, run up and down stairs - as this heightens the fizzing.
4. Always check the expiration date of the peroxide. If it is old it doesn't fizz very well!
Given that diarrhea and vomiting are expected in the next day, I would suggest the following for patients that I had examined:
1. Because we expect vomiting/diarrhea, you should WITH-HOLD FOOD! Do not offer anything for 12- 24 hours. This gives the intestines a chance to rest and heal.
2. When he is fasting, he can have lots of clear fluids. So, water is fine, but also he can have pedialyte, Gatorade, apple juice diluted 50:50 with water, or chicken or beef broth diluted 50:50 with water. Give the fluids in small amounts frequently. For a dog this size that means about 1/2-3/4 cup an hour.
3. After 12-24 hours if the diarrhea has stopped, you can start your dog back on a bland diet. For patients that I see, I recommend a mixture of 75% cooked white rice, and 25% low fat protein. For the protein you could use extra lean ground beef, boiled with the fat scooped off, or chicken breast boiled with fat scooped off or even scrambled egg cooked without fat in the microwave. Feed small frequent meals. For a dog this size, I would suggest 1/2 cup every 3 to 4 hours.
4. After 1-2 days on the rice mix, you would gradually change your dog back to the normal diet and food. So, on day 3, give the rice mixture, but bigger meals, spaced further apart. On day 4, mix a little tiny bit of the normal food in there, and decrease the frequency so it is down to 3 meals or so. And so on.
5. Keep your dog as quiet as possible - just out to relieve himself and back in.
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The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would strongly advise contacting your regular veterinarian.
Best wishes, Fiona