According to WebMD, an overdose of calcium carbonate, the active ingredient in Tums, causes the following symptoms: nausea, vomiting, loss of appetite, mood swings, headaches, weakness and dizziness. People who believe they have overdosed on Tums should seek emergency treatment.
The National Institutes of Health MedlinePlus website advises that a person experiencing an overdose of calcium carbonate should not induce vomiting. It also states that calcium carbonate is not very toxic, so full recovery from an overdose is generally expected. It is unusual for someone to die from an overdose of calcium carbonate, however chronic misuse can cause long-term problems.