Common Names

  • Calcium carbonate
  • calcium citrate
  • calcium gluconate

For Patients & Caregivers

Bottom Line: Adequate calcium intake can help prevent osteoporosis and bone fracture. However, calcium supplements may increase the risk of cardiovascular events. Studies of cancer prevention effects of calcium yielded mixed results.
Calcium plays several essential roles in the human body. It is part of the main structural element in bone, and bone also acts as a calcium reservoir so that that blood calcium levels are maintained at stable levels. This is necessary because of the many roles calcium plays in cells, including muscle contraction, nerve transmission, blood clotting, and enzyme reactions. When calcium intake and bodily estrogen levels are low, the body takes calcium from bones and uses it for muscle, neurologic, and enzyme reactions. As a result, osteoporosis can occur. But an analysis of 15 clinical trials suggests an increased risk of heart problems with calcium supplements.

Inadequate blood calcium levels are linked to high blood pressure. This is known because high blood pressure is often associated with abnormally high calcium levels in the urine, which indicates that too much calcium is being lost from the blood, and from other data.

  • To prevent cancer
    Many clinical studies have been performed in humans, but the consistent effects of dietary and supplemental calcium on cancer have not been determined.
  • To lower high blood pressure
    Most clinical trials have found that calcium from the diet and from supplements can have a small effect on lowering both systolic and diastolic blood pressure.
  • To prevent the progression of osteoporosis
    Several major clinical trials have found that calcium supplementation prevents bone loss, especially in people over the age of 50. Reduced bone loss may also prevent fractures.
  • To prevent premenstrual syndrome (PMS)
    Several clinical trials support this use.
  • To reduce lead levels in children with lead poisoning
    One clinical trial showed no effect.

Premenstrual syndrome (PMS)
A randomized controlled trial was conducted to study the effect of calcium supplements on premenstrual and menstrual symptoms in 497 women. The women were given either 1200 mg of calcium carbonate or a placebo pill every day for three months. By the third menstrual cycle, the women taking calcium reported that their symptoms were half as bad as they used to be, while the group taking the placebo reported only a 30% decrease in symptoms. This study supports the use of calcium to treat premenstrual syndrome symptoms.

For three years, 389 men and women over 65 were randomly assigned to take 500 mg/day calcium plus 700 IU/day vitamin D3 (cholecalciferol) or similar placebo pills. The group taking calcium and vitamin D showed significantly less loss of total body bone mineral density in the second and third year of treatment, and had fewer nonvertebral fractures than the placebo group. While this study does not imply a benefit of calcium alone, it does support the use of calcium and vitamin D3 together to improve bone density in older men and women.

  • You have hypothyroidism, high blood calcium levels, or low blood phosphate levels (Consult your doctor before starting calcium supplements).
  • You are taking bisphosphonates (Calcium may reduce the absorption of etidronate. Calcium should be taken two hours before or one hour after taking this medicine).
  • You are taking cardiac glycosides such as digoxin (Calcium may increase the risk of cardiac arrhythmia).
  • You are undergoing treatment with tamoxifen (Calcium may increase the risk of hypercalcemia, or abnormally high calcium levels in the blood. This can increase the risk of kidney stones).
  • Are taking fluoroquinolone or tetracycline antibiotics (Calcium may reduce the absorption of these antibiotics into the bloodstream. Calcium should be taken two hours before or one hour after taking these medicines).
  • Constipation
  • Flatulence
  • Chalky taste and dry mouth
  • Nausea occurs rarely
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For Healthcare Professionals

Oscal®, Tums®, Caltrate®, Citracal®

Calcium is an essential mineral responsible for many physiological functions in the body. It is stored in the bones and helps maintain bone structure. Calcium also plays an important role in cell signaling and muscle contraction. Dietary calcium can be found in dairy products, nuts and fish. Pregnant and breast-feeding women, children, the elderly, and post-menopausal women may require higher intake (1)(2)(3). Natural supplements are derived from minerals, oyster shells and occasionally corals.

Calcium has been studied for its use in treating a range of conditions, including osteoporosis, bone density loss (4)(5), premenstrual syndrome (PMS) (6)(7), preeclampsia (8), lead poisoning (9), and colon (10) and prostate cancer. Calcium supplementation may have beneficial effects on cardiovascular risk factors such as hypertension (11)(12)(13) but this effect has not been shown in postmenopausal women (14). Because low dietary intake of calcium may reduce bone mass and increase the risk of osteoporosis, several studies have addressed the effects of calcium with or without vitamin D supplementation on bone mineral density (15)(16).

Studies of calcium supplementation with or without vitamin D3 and fracture prevention in the elderly offer conflicting data (17)(18).
In postmenopausal women, neither hip fracture (19) nor physical decline (20) was reduced. But some studies found benefits in adults in over age 50 and in premenopausal women (21)(22). However, according to the latest statement from the U.S. Preventive Services Task Force (USPSTF), current evidence is insufficient to assess the balance of the benefits and harms of combined vitamin D and calcium supplementation for the primary prevention of fractures in premenopausal women or in men. Evidence is also lacking to determine the balance of the benefits and harms of daily supplementation with greater than 400 IU of vitamin D3 and greater than 1000 mg of calcium for the primary prevention of fractures in noninstitutionalized postmenopausal women. The USPSTF recommends against daily supplementation with 400 IU or less of vitamin D3 and 1000 mg or less of calcium for the primary prevention of fractures in noninstitutionalized postmenopausal women (35).

Data also suggest that increasing dietary intake of calcium does not benefit cardiovascular health (34), while the supplemental form may increase the risk of cardiovascular events (31) and myocardial infarction(34).

Calcium is thought to have a protective effect against cancer. However, the Women’s Health Initiative study found no association between calcium and vitamin D supplementation and reduced risk of colorectal cancer (24) or invasive cancer and mortality (33) in postmenopausal women. Subsequent reanalysis suggested that estrogen therapy may interact with the supplements (25). Data from a randomized trial indicate a significant reduction in risk of cancer in postmenopausal women following vitamin D and calcium supplementation (26). In men, calcium intake may reduce risk of colorectal cancer (26), but may increase the risk of prostate cancer (27).

Studies also showed that supplemental calcium blocks absorption of lead from other foods (23). In children with lead poisoning, calcium supplementation had no effect on blood lead levels (9). Calcium supplements may cause constipation and gastrointestinal upset. Some calcium salts can neutralize gastric acid and can interfere with absorption of prescription medications, (28); Caffeine may lower calcium absorption and increase urinary calcium excretion (29). A few cases of kidney stones have been reported at high doses.

A recent report from the Institute of Medicine recommends a Dietary Allowance at 1,000mg/day for bone health. Adolescents and pregnant/lactating women may need up to 1,300 mg/day (32). Patients should consult with their physicians if more calcium is needed for health maintenance.

Milk, yogurt, cheese, egg, bread, salmon, prawns, sardines, shrimp, broccoli, spinach, spring greens, baked beans, chickpeas, lentils, kidney beans, soya beans, tofu, orange, almonds, sesame seeds and fortified cereals, orange juice and soy milk

  • Cancer prevention
  • Hypertension
  • Osteoporosis
  • Premenstrual syndrome

Most of calcium reserve in humans is found in the bones where calcium helps to maintain the structural. Calcium is recycled through the actions of osteoblasts and osteoclasts, replacing and resorbing bone minerals, respectively. Low estrogen and calcium levels can increase the number of osteoclasts to convert bone minerals to extracellular calcium. Calcium deficiency over time may manifest as skeletal weakness or fractures. Tumor metastasis can cause elevation of serum calcium level.

Calcium also plays an important role in a variety of muscular, neurological, hormonal, and enzymatic reactions throughout the body. Primarily a second messenger transmitting signals between the plasma membrane and the intracellular bodies. Low serum calcium concentration may increase intracellular calcium concentration in vascular smooth muscle cells, which may increase vascular resistance. Calcium is also essential in the functions of clotting factors and adhesion molecules.

Calcium absorption occurs in the duodenum, jejunum and ileum through a paracellular, unregulated, nonsaturable mechanism when intake levels are high and through a vitamin D-assisted, transcellular, saturable, active-transport mechanism when intake is low. Passive diffusion may occur in the colon. Calcium absorption is affected by a variety of substances. Alcohol, aluminum-containing antacids, laxatives, oxalic acid (found in cauliflower, spinach and rhubarb), high sodium intake and caffeine may all reduce calcium absorption. Phyates and oxylates greatly reduce calcium bioavailability by forming stable indigestible complexes. Increased protein intake can increase calcium absorption. The efficiency of calcium absorption increases during childhood, adolescence, pregnancy and lactation and decreases in the elderly.
Nearly all of the body’s calcium is stored in the bones and teeth. The ionized, physiologically-active form of calcium found in the blood is controlled by parathyroid hormone, calcitonin and vitamin D among other hormones. Anticonvulsants and corticosteroids may reduce serum calcium levels.
Calcium excretion occurs in the urine, feces, sweat and breast milk. The amount excreted varies with the quantity absorbed and the degree of bone loss. Fecal loss may be as high as 90% of ingested calcium. Strenuous athletic exertion can increase dermal loss. Urinary calcium loss is regulated by parathyroid hormone (PTH), calcitonin and 1,25-dihydroxyvitamin D and also depends on protein and sodium intake. Thiazide diuretics may reduce calcium excretion. Loop diuretics may increase excretion of calcium.

Calcium may interfere with the absorption of iron, fluoride and zinc.
High consumption of calcium has been associated with an increased risk of prostate cancer and milk-alkali syndrome.
An independent lab found that some calcium supplements have been shown to contain excessive amounts of lead.

Patients who have hypothyroidism, high serum calcium level or low serum phosphate level should consult their physicians before taking calcium supplements.

Common: Constipation, flatulence, chalky taste and dry mouth
Rare: Nausea

Proton Pump Inhibitors: May significantly reduce calcium absorption (33)
Bisphosphonates: Calcium may reduce absorption of etidronate.
Cardiac glycosides: Calcium may increase risk of cardiac arrhythmia.
4-Quinolones: Calcium may reduce absorption of 4-quinolones.
Tamoxifen: Calcium may increase the risk of hypercalcemia.
Tetracyclines: Calcium may reduce absorption of tetracyclines.

Decreased blood pressure

Tang BM, et al. Use of calcium or calcium in combination with vitamin D supplementation to prevent fractures and bone loss in people aged 50 years and older: a meta-analysis. Lancet 2007;370(9588):657-666.
In a meta-analysis of 29 randomized clinical trials with 63,897 participants over the age of 50, the effects of calcium alone or with vitamin D on bone loss and fracture risk were determined. Calcium and calcium with vitamin D was associated with a reduction in fracture risk (12%) and bone loss at the hip and spine. Study compliance was associated with increased risk reduction. Furthermore, in studies using calcium alone, greater treatment effects were found when a minimum dose of > 1200 mg calcium was used, and in studies using calcium and vitamin D, >800 IU Vitamin D doses were needed to achieve maximum benefits. Finally, fracture risk reduction was not significantly affected by the addition of vitamin D. Therefore, for individuals over the age of 50, calcium supplementation alone or with vitamin D reduced bone loss and fracture risk.

Jackson, RD, et al. Calcium plus Vitamin D supplementation and the risk of fractures. N Engl J Med 2006; 354(7): 669-83.
Data obtained from the Women’s Health Initiative study were analyzed to determine the efficacy of calcium and vitamin D supplementation for reducing hip fractures. The study involved 36,282 postmenopausal women who were randomized to receive 1000mg of calcium as calcium carbonate with 400 IU vitamin D3 daily or placebo for seven years. There was no significant reduction in hip fractures. But there was a small but significant improvement in hip bone density and an increased risk of kidney stones.

Wactawski-Wende J, et al. Calcium plus Vitamin D supplementation and the risk of colorectal cancer. N Engl J Med 2006; 354(7):684-96.
Data obtained from the Women’s Health Initiative study were analyzed to determine the association of calcium and vitamin D supplementation with reduced risk of colorectal cancer. The study invovled 36,282 postmenopausal women who were randomly assigned to receive 500 mg calcium as calcium carbonate with 200 IU of vitamin D3 twice daily or a matching placebo for seven years. Results showed no effect of supplementation on the incidence of colorectal cancer.

These data conflict those of several earlier studies that suggest a benefit of increased calcium and vitamin D supplementation with reduced risk of cancer. Researchers point to the drawbacks of earlier studies since most were observational in nature and prone to bias. They also indicate the limitations of the present study that may have contributed to these findings. These include the timing of supplementation and the short follow-up period as colorectal cancer can be latent for 10-20 years. In addition, participants in the study were taking calcium and vitamin D supplements prior to enrollment. So the higher intake of supplements during the study period may have also affected the rates of colorectal cancer.

Subsequent reanalysis of the data found that estrogen therapy interacts with calcium and vitamin D supplementation. Specifically, increased risk of developing colorectal cancer was detected in women receiving calcium, vitamin D, and estrogen therapy and reduced risk was seen in those receiving only calcium and vitamin D. Although the results did not reach statistical significance, this study suggests that calcium and vitamin D supplementation may negatively interact with estrogen therapy, increasing rather than decreasing the risk of colorectal cancer.

Grant AM, et al. Oral vitamin D3 and calcium for secondary prevention of low-trauma fractures in elderly people (Randomized evaluation of calcium or vitamin D, record): a randomised placebo-controlled trial. Lancet 2005;365:1621-28.
In this study 5,292 people, aged 70 years or older who had previous low-trauma fracture, were randomized to receive 800 IU daily oral vitamin D3, 1000 mg calcium, 800 IU oral vitamin D3 combined with 1000 mg calcium daily, or placebo. The participants were followed up for between 24 and 62 months for new fractures. Researchers found that 13% of all the participants had a new low-trauma fracture. However, there was no significant difference in the occurrence of fractures between any of the groups. These results suggest that oral supplementation with calcium used alone or in combination with vitamin D3 is not effective in preventing risk of secondary fractures in elderly people.

  1. Ross EA, Szabo NJ, Tebbett IR.Lead content of calcium supplements. JAMA 2000;284:1425-9.

  2. Levine RJ, et al. Trial of calcium to prevent preeclampsia. N Engl J Med 1997;337:69-76.

  3. Hallberg L. Does calcium interfere with iron absorption? Am J Clin Nutr 1998;68:3-4.

  4. Minihane AM, Fairweather-Tait SJ. Effect of calcium supplementation on daily nonheme-iron absorption and long-term iron status. Am J Clin Nutr 1998;68:96-102.

  5. Heaney RP, Rafferty K. Carbonated beverages and urinary calcium excretion. Am.J Clin Nutr 2001;74:343-7.

  6. White E, Shannon JS, Patterson RE. Relationship between vitamin and calcium supplement use and colon cancer. Cancer Epidemiol Biomarkers Prev 1997;6:769-74.

  7. Mason, P. Dietary Supplements, 2nd ed. London (England): Pharmaceutical Press; 2001.

  8. Power ML, et al. The role of calcium in health and disease. Am J Obstet Gynecol 1999;181:1560-9.

  9., LLC. (last accessed on 7/7/2005).

  10. Janakiraman v, et al. Calcium supplements and bone resorption in pregnancy: a randomized crossover trial. Am J Prev Med 2003 Apr;24(3):260-4.

  11. Cook JD, et al. Calcium supplementation: effect on iron absorption. Am J Clin Nutr 1991 Jan;53(1):106-11.

  12. Markowitz ME, Sinnett M, Rosen JF. A randomized trial of calcium supplementation for childhood lead poisoning. Pediatrics. 2004 Jan;113(1 Pt 1):e34-9.

  13. Jackson RD, et al. Calcium plus vitamin D supplementation and the risk of fractures. N Engl J Med 2006; 354(7):669-83.

  14. Wactawski-Wendi J, et al. Calcium plus vitamin D supplementation and the risk of colorectal cancer. N Engl J Med 2006; 354(7):684-96.

  15. O’Connell MB, Madden DM, Murray AM, et al. Effects of proton pump inhibitors on calcium carbonate absorption in women: a randomized crossover trial. Am J Med 2005;118:778-781.

  16. Lappe JM, Travers-Gustafson D, Davies KM, et al. Vitamin D and calcium supplementation reduces cancer risk: results of a randomized trial. Am J Clin Nutr 2007;85:1586-91.

  17. Chlebowski RT, Johnson KC, Kooperberg C, et al. Calcium Plus Vitamin D Supplementation and the risk of breast cancer. J Natl Cancer Inst. 2008 Nov 11.

  18. Institute of Medicine of the National Academies. Dietary Reference Intakes for Calcium and Vitamin D. Consensus Report, Nov 30, 2010.

  19. Virginia A. Moyer, ; Vitamin D and Calcium Supplementation to Prevent Fractures in Adults: U.S. Preventive Services Task Force Recommendation Statement. Annals of Internal Medicine. 2013 Feb. [Epub ahead of print]

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