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Ingredient Type: Mineral

Also Known As: CaCo3, Calcium carbonate, Ca3(C6H5O7)2, Calcium citrate

Calcium is the fifth most abundant element by mass in the Earth’s crust.  It is essential for living organisms, functioning as a signal for many cellular processes, and is used in mineralization of bone, teeth, and shells.  Calcium is found in many foods, including dairy products, nuts, broccoli, leafy greens, and tofu.  Absorption of calcium is boosted by Vitamin D and Magnesium.

The two main forms of supplemental calcium are calcium carbonate (CaCo3) or calcium citrate (Ca3(C6H5O7)2).  Calcium carbonate is an odorless, tasteless powder or crystal that occurs in nature.  It is commonly found in rocks and the minerals: calcite and aragonite and is the main component of pearls and the sells of marine organisms, snails, and eggs.  Calcium carbonate is also the active ingredient in agricultural lime and is created when calcium ions in hard water react with carbonate ions to create limescale.

Calcium citrate also occurs naturally.  It is most commonly found in citrus fruits, other plants, and as the mineral Earlandite.

Both of these forms of calcium can be absorbed by the human body.  Calcium carbonate is, however, dependent upon stomach acid for absorption and does not, therefore, absorb well on an empty stomach.  Calcium citrate can be absorbed equally well on a full or empty stomach.  The citrate form also seems to be better tolerated by people with inflammatory bowel disease, who underproduce stomach acids, or who are sensitive to antiacids (1).


Calcium carbonate is a common over the counter medicine used as a calcium supplement or an antacid.  It is used to treat symptoms caused by too much stomach acid such as heartburn, upset stomach or indigestion.  It works as an antacid by neutralizing some of the acids in the stomach (2).  Other conditions treated with calcium carbonate are: osteoporosis, low calcium, indigestion, heartburn, renal osteodystrophy (2).

Calcium citrate is used for most of the same medicinal purposes as calcium carbonate.  It is not, however, used as an antacid.


  • Recent investigations suggest that calcium supplementation may cause a lower arterial pressure in hypertensive individuals (3).
  • Calcium may have a beneficial or neutral effect on cardiovascular risk factors such as blood pressure, cholesterol, weight, and diabetes (4).
  • In vitro and in vivo laboratory studies suggest that calcium may be involved in cardiovascular disease development through multiple pathways, including blood cholesterol, insulin secretion and sensitivity, vasodilation, inflammatory profile, thrombosis, obesity, and vascular calcification (5).


Calcium is likely safe when used at or below recommended doses.  Excessive calcium, however, is possibly unsafe and can lead to a serious condition known as milk-alkali syndrome.  During the past 15 years, this condition has been reported in women taking calcium supplements well above the recommended range of 1.0 to 1.6 g daily.  Excessive calcium intake can also lead to hypercalcemia and associated vomiting, abdominal pain, irregular heart rhythm, and altered mental status (6).

Calcium intake, which includes dietary intake from food plus supplements, should stay within the recommended range.  Doses above the daily tolerable upper intake level (2.5 g for ages 19-50 and 2.0 g for ages 51+) lead to increased risk of the conditions mentioned above.



  • Calcium can bind to certain medicines and decrease their absorption or bioavailability.  Because of that, calcium should not be taken at the same time as Doulutegravir, Bictegravir, or Elvitegravir/Vitekta (7).


  • Calcium can bind to certain medicines and decrease their absorption or bioavailability.  Because of that, calcium should not be taken at the same time as Levothyroxine, Quinolone or Tetracycline antibiotics, or Sotalol.
  • Some research indicates that calcium can affect the heart.  Caution is, therefore, advised in taking calcium with certain other medications that also affect the heart like: Digoxin, Verapamil, and Diltiazem.
  • Because citrate can increase the absorption of certain metals, calcium citrate should not be taken with aluminum salts.  Calcium carbonate, however, does not interact with these salt in the same way (7).

For a more comprehensive list of drugs known to interact with calcium see the website.


When taken at or below recommended doses calcium has been reported to cause belching and flatulence.  Other side effects like acid rebound or constipation have not yet been substantiated by scientific evidence (7).


  1. Calcium. National Institutes of Health Office of Dietary Supplements website. Accessed March 3, 2018.
  2. What conditions does calcium carbonate tablet treat? WebMD. Accessed February 2018.
  3. Bloomfield RL, Young LD, Zurek G, Felts JH, Straw MK. Effects of oral calcium carbonate on blood pressure in subjects with mildly elevated arterial pressure. Hypertens Suppl. 1986 Dec;4(5):S351-354
  4. Waldman T, Sarbaziha, Bairey Merz CN, Shufelt C. Calcium supplements and cardiovascular disease. Am J Lifestyle Med. 2015;9(4):298-307. doi:10.1177/1559827613512593.
  5. Rautiainen S, Wang L, Manson JE, Sess HD. The role of calcium in the prevention of cardiovascular disease-a review of observational studies and randomized clinical trials. Curr Atheroscler Rep. 2013;15(11):362. doi:10.1007/s11883-013-0362-4.
  6. Gabriely I, Leu JP, Barzel US. Clinical problem-solving, back to basics. New Eng J of Medicine. 2008;358(18): 1952–1956. doi:10.1056/NEJMcps0706188.
  7. Calcium. Natural Medicines Comprehensive Database. Accessed March 12, 2018.

See the Michigan Medicine Health Library entry for calcium, the MedlinePlus entry for calcium, or the entry for calcium for more information.