Calcium is the most abundant mineral in the body and makes up much of the structure of bones and teeth; it is also needed for muscle, nerve and blood-vessel function. Calcium supplements are used to cover low intake from food, often for bone health in older adults, usually together with vitamin D.
Also known as: calcium carbonate, calcium citrate.
ODS lists a Recommended Dietary Allowance of 1,000 mg a day for adults 19 to 50 and men 51 to 70, and 1,200 mg for women 51 and up and everyone over 70. The Tolerable Upper Intake Level is 2,500 mg a day for adults 19 to 50 and 2,000 mg from age 51. Calcium carbonate is 40% calcium by weight and calcium citrate 21%; the label lists elemental calcium.
It is sold in the US as a dietary supplement. Under the 1994 dietary supplement law (DSHEA), FDA does not approve dietary supplements before they are marketed, and the maker, not FDA, is responsible for safety and labeling. Calcium is also an ingredient in some over-the-counter antacids. FDA has approved a health claim that supplements with calcium and vitamin D may reduce the risk of osteoporosis in older adults, though ODS notes not all trials found better bone health or fewer fractures.
ODS says the share absorbed falls as the amount taken at once rises (about 36% of a 300 mg dose and 28% of a 1,000 mg dose), and absorption is highest at 500 mg or less. Calcium carbonate is absorbed better with food; calcium citrate can be taken without food. Gas, bloating and constipation are common, more so with carbonate. High supplemental intake might raise the risk of kidney stones, and some studies (not all) link calcium supplements to heart disease.
Medicine timing, per ODS: take levothyroxine at least 4 hours apart from calcium carbonate; take quinolone antibiotics such as ciprofloxacin 2 hours before or after calcium; take dolutegravir 2 hours before or 6 hours after calcium. Lithium plus calcium supplements may raise the risk of high blood calcium.
We could not find a single published half-life or time to peak for calcium that applies to a supplement dose, so we do not list one. Blood calcium is tightly controlled by hormones (with vitamin D needed for active absorption), so a supplement dose does not show up as a simple curve.
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