[2][3] By mouth they are used to treat and prevent low blood calcium, osteoporosis, and rickets.

[7][8] Versions are also sold together with vitamin D.[3] In 2020, the combination, calcium/vitamin D was the 215th most commonly prescribed medication in the United States, with more than 2 million prescriptions.

[9][10] In healthy people, calcium supplementation is not necessary for maintaining bone mineral density, and carries risks that outweigh any benefits.

[11] Calcium intake is not significantly associated with hip fracture risk in either men or women.

[18] Excessive consumption of calcium carbonate antacids/dietary supplements (such as Tums) over a period of weeks or months can cause milk-alkali syndrome, with symptoms ranging from hypercalcemia to potentially fatal kidney failure.

[20] A 2023 systematic review found that calcium supplementation is not associated with myocardial infarction, stroke, heart failure admission, and cardiovascular/all-cause mortality.

[24] Thus, people taking both calcium and thyroxine run the risk of inadequate thyroid hormone replacement and thence hypothyroidism if they take them simultaneously or near-simultaneously, because significant amount of T4 is adsorbed to calcium carbonate depending on gastric pH levels, preventing absorption of T4 at the intestinal level.

Calcium absorption varies with serum 25-hydroxyvitamin D [25(OH)D] levels and is also influenced by dietary factors and genetics.

Several factors can influence the rate of urinary calcium loss, including the intake of caffeine, protein, and sodium and low estrogen levels.

[49] For US dietary supplement and food labeling purposes, the amount in a serving is expressed in milligrams and as a percent of Daily Value (%DV).