While it is generally considered safe to give a dog TUMS, this medication is not an ideal treatment for a canine's upset stomach. Even though a moderate dose of the active ingredient in TUMS is unlikely to cause any major negative effects in a dog, it is also unlikely to provide any relief due to the way that dogs digest food.
The active ingredient in TUMS and many other antacids is a chemical called calcium carbonate. Calcium carbonate is not only a medication, it also occurs widely in nature as one of the main substances that make up limestone.
Calcium carbonate treats an upset stomach because of the effects it has on stomach acid. Calcium carbonate is a basic substance. This means it has a high pH, which is the opposite of stomach acid, a substance with a very low pH. When a person or animal is given calcium carbonate it goes to work by neutralizing stomach acid. This happens due to a chemical reaction that takes place between calcium carbonate and hydrochloric acid, the main constituent of stomach acid. When this reaction happens, calcium carbonate and hydrochloric acid switch their chemical partners, forming calcium chloride and water. Calcium chloride is insoluble and passes through the digestive tract, while the water formed by the reaction results in a healthier pH level.
Unfortunately, dogs digest foods much more quickly than humans do, so the chemical reaction between calcium carbonate and stomach acid does not occur fast enough to treat the animal's upset stomach. By the time calcium carbonate treats the symptom, the dog has typically already digested and passed whatever food was causing the problem in the first place, making this a harmless but relatively ineffective treatment for the animal's ailment.