You need calcium for proper nerve and muscle function and for forming strong bones. People worried that they aren't getting enough calcium from their diet sometimes take calcium supplements, most often in the form of calcium carbonate. These supplements can help you increase your calcium intake, but don't exceed the tolerable upper intake level of 2,500 milligrams per day unless your doctor recommends it because this can lead to potentially serious side effects.
Constipation is one of the most common side effects of calcium carbonate, affecting up to 10 percent of people, and can occur even with smaller doses of this supplement. Taking too much calcium carbonate could also interfere with your ability to absorb other minerals, such as iron and zinc. A calcium carbonate overdose may cause abdominal pain, nausea, diarrhea and vomiting. Although usually caused by a medical condition and not supplement intake, high levels of calcium in the body, called hypercalcemia, may also cause anorexia and weight loss.
The kidneys play a role in clearing extra calcium from the body, so taking too much calcium carbonate over long periods of time can harm your kidneys, potentially causing impaired kidney function, especially if you already have kidney problems. Taking too much calcium carbonate may also increase your risk for kidney stones.
Soft Tissue Issues
Having high levels of calcium over a period of time may cause your soft tissues, such as those in your organs, to become calcified, or hardened, because of calcium deposits. This interferes with the functioning of these organs but isn't likely to occur unless you have impaired kidney function, according to the National Academies Press publication "Dietary Reference Intakes for Calcium and Vitamin D."
Other Potential Effects
Other potential effects of an overdose of calcium carbonate include headache, confusion, bone pain, depression, muscle twitching, irregular heartbeat and coma. Taking too much calcium carbonate may also increase your risk for heart disease and prostate cancer, but the evidence for this is still preliminary and conflicting, according to the Office of Dietary Supplements.