What Is Calcium Carbonate?

FDA regulations at 21 CFR 184.1191 say calcium carbonate is used in food with no limitation other than current good manufacturing practice. The FDA over-the-counter antacid rule lists calcium, as carbonate or phosphate, as an antacid active ingredient with a maximum daily dosage limit of 160 mEq of calcium, for example 8 grams of calcium carbonate.

The NIH Office of Dietary Supplements says absorption of calcium supplements is generally greater when they are taken with food, regardless of whether stomach acid is low, and that some people who take calcium supplements have gas, bloating or constipation, with calcium carbonate appearing to cause more of these side effects than calcium citrate, especially in older adults who have lower levels of stomach acid. It lists the tolerable upper intake level for calcium as 2,500 mg a day for adults aged 19 to 50 and 2,000 mg a day for adults 51 and older.

Too much can cause milk-alkali syndrome. A 2026 case report in Clinical Practice and Cases in Emergency Medicine describes milk-alkali syndrome as the triad of high blood calcium, metabolic alkalosis and acute kidney injury resulting from excessive intake of calcium and absorbable alkali.