Constipation, bloating, and gas are common with calcium supplements (especially carbonate salt). Calcium absorption is impaired in achlorhydria (common in the elderly). Hypercalcemia and hypercalciuria are most likely to occur in hypoparathyroid patients receiving high doses of vitamin D. Use caution when administering calcium supplements to patients with history of kidney stones. Calcium administration interferes with absorption of some minerals and drugs. It is recommended to administer vitamin D concomitantly for optimal calcium absorption. Taking calcium (≤500 mg) with food improves absorption. May produce decreased serum magnesium determinations.