- Calcium carbonate
- Calcium citrate
- Calcium gluconate
For Patients & Caregivers
Tell your healthcare providers about any dietary supplements you’re taking, such as herbs, vitamins, minerals, and natural or home remedies. This will help them manage your care and keep you safe.
What is it?
Calcium is a mineral that you need for many bodily functions. It also helps build and maintain healthy bones. It’s found in foods such as dairy products, dark greens, legumes, nuts, and fish.
If you don’t get enough calcium from food alone, your healthcare provider may recommend you take calcium supplements. Calcium supplements come as tablets or capsules.
What are the potential uses and benefits?
Calcium is used to:
- Prevent cancer
- Lower high blood pressure
- Decrease risk of heart disease
- Prevent bone loss (osteoporosis)
Calcium also has other uses that haven’t been studied by doctors to see if they work.
Calcium that you get from food is safe. Talk with your healthcare provider before taking supplements. Supplements are stronger than the calcium you get from food. They can also interact with some medications and affect how they work. For more information, read the “What else do I need to know?” section below.
What are the side effects?
Side effects of using calcium may include:
- Constipation (having fewer bowel movements than usual)
- Chalky taste in the mouth
- Dry mouth
- Higher risk of urinary stones (in long-time users)
- Nausea (feeling like you’re going to throw up)
- Increases risk of stroke when taking high-dose calcium supplements
What else do I need to know?
- Talk to your healthcare provider before taking calcium supplements if you have hypothyroidism (less thyroid hormone in the blood), high blood calcium levels, or low blood phosphate levels. Calcium can worsen these conditions.
- Talk to your doctor before taking calcium supplements if you’re taking medications such as digoxin (Lanoxin®) to treat a heart condition. Calcium may increase the risk of irregular heartbeat.
- Talk to your healthcare provider if you’re taking tamoxifen (Nolvadex or Soltamox™) as part of your cancer treatment. Calcium may increase the risk of abnormally high levels of calcium in the blood. This can increase the risk of kidney stones.
For Healthcare Professionals
Calcium is an essential mineral responsible for many physiological functions. It helps maintain bone structure and plays an important role in cell signaling and muscle contraction. Dietary sources of calcium include dairy products, dark greens, legumes, nuts and fish. Natural supplements are derived from minerals, oyster shells and occasionally corals.
Calcium has been studied for a range of conditions including cardiovascular disease (1) (2) (3) (4) (5), osteoporosis (6), bone density loss (7), fracture prevention (8), premenstrual syndrome (9), pre-eclampsia (10) (11) (87), lead poisoning (12), and various cancers (13) (14) (15).
Long-term supplement use was associated with an increased risk of coronary artery calcification (77) and may lead to cardiovascular disease (CVD) (1) (2) (3) (5), although findings remain debatable (16) (17) (18). In a Women’s Health Initiative sub-study, calcium plus vitamin D supplementation and hormone therapy yielded greater reductions in LDL-C than either intervention alone or placebo (81) but supplementation did not always modify the effects of hormonal therapy on CVD events (88). Calcium and vitamin D supplementation may also benefit postmenopausal women who do not have pre-existing risk factors such as coronary heart disease, diabetes, or hypertension (19); and large long-term studies in women did not find adverse cardiovascular associations with calcium supplementation (4) (20), but a meta analysis reported an increase in the risk of CVD (94). High intake of supplements also increased cardiovascular mortality in men (21). Another study found supplementation to be associated with dementia in women following stroke (75), but dietary intake of vitamin D and calcium were reported useful in maintaining cognitive performance in older females (90).
There is also conflicting evidence surrounding the benefits of calcium or calcium plus vitamin D supplements for bone health (22) (23) (24) (42) (53) (82). The US Preventive Services Task Force did not find any associations between calcium plus vitamin D supplementation and reduced falls or fracture incidence in community-dwelling older adults without known deficiencies, osteoporosis, or history of fracture (83) (84). In postmenopausal women, neither dietary calcium (25) nor calcium plus vitamin D supplements (26) reduced risk of hip fracture, although the latter study found significant reductions at 7 years (27). Subgroup analysis further suggested that supplementation along with estrogen therapy conferred additive protection against hip fractures (28). However, there appear to be no preventive benefits with supplementation for bone mineral density loss in breast cancer patients (7), in older adults (78), or in healthy men (79). But in a study of pediatric cancer survivors, hypocalcemia induced by standard osteoporotic medication was treated with continuous calcium supplementation (91). Calcium plus vitamin D also reduced bone density loss associated with antiretroviral therapy in HIV patients (72) (92).
In cancer settings, both dietary and supplemental calcium were linked with lower risk of colorectal cancer (13) (29) (93) (99), with dietary calcium more beneficial in reducing the risk of incidence and recurrence of colorectal and advanced adenomas (95). Calcium intake was associated with increased risk for prostate cancer (30) (31); notably a genetic disposition for high intestinal absorption of calcium was reported in African-American men (32) (33). Long-term studies suggest risks associated with calcium in prostate cancers are less strong when phosphorus intake is also considered (34) (35). Data also suggest benefits against cutaneous metastases (100).
In women, studies on calcium plus vitamin D supplements for reduced risk reported positive association with ductal carcinoma in situ (DCIS) (96) but conflicting data for colorectal cancers (38) (85), all cancer types (36) (86), and mortality (26) (37) (101). Other trials suggest benefits against hematologic (80) but not invasive cancers (39).
Available evidence is insufficient to assess benefits of calcium plus vitamin D supplementation for those receiving androgen deprivation therapy, aromatase inhibitors, or undergoing chemotherapy-induced menopause (7) (43) (44) (97) (98). Patients should consult with their oncologists or oncology healthcare teams, especially since calcium supplements can interact with a number of prescription medications.
A balanced diet containing sodium, potassium, magnesium, calcium and vitamin D is important to reduce risks for hypertension as well as cardiovascular and cerebrovascular events (40). But the risk-benefit ratio of supplementation is likely to vary, depending on dietary calcium intake, sex, age, ethnicity, and individual risks for cardiovascular disease and osteoporosis (41) (73). In adults with normal calcium levels, genetic predisposition to higher serum levels does not affect bone mineral density nor protect against fractures (89). Intake of high-dose calcium supplements (>1 g/day) may actually increase the risk of ischemic stroke (76), and calcium plus vitamin D supplementation may elevate the risk for kidney stones (42) (48) (53) (84).
Milk, yogurt, cheese, egg, bread, salmon, prawns, sardines, shrimp, broccoli, spinach, kale, collards, spring greens, baked beans, chickpeas, lentils, kidney beans, soya beans, tofu, orange, almonds, sesame seeds and fortified cereals, orange juice and soy milk
Purported Uses and Benefits
- Cancer prevention
- Cardiovascular disease
Mechanism of Action
Calcium plays an important role in a variety of muscular, vascular, neurological, hormonal, and enzymatic reactions. Calcium reserves are found mostly in the bones, helping to maintain skeletal structure.
The association of supplemental but not dietary calcium with increased cardiovascular risk could be related to their differing, acute effects on serum calcium (45). Supplementation may fail to compensate for renal calcium loss, resulting in increased circulatory calcium that could lead to coronary artery deposits (46). Reduced dietary intake may cause calcium depletion in membrane storage sites decreasing stability of vascular smooth muscle cell membranes, as optimal concentrations stabilize these membranes, inhibit calcium entry into cells, and reduce vasoconstriction (47). Gastrointestinal events associated with calcium supplements may also account for an increase in self-reported cardiovascular events (18).
Observed associations of dairy with overall prostate cancer risk may be related to the modulation of vitamin D metabolism by calcium and phosphorus (15) (31). Saturated fat in whole dairy correlating with higher C-peptide concentrations, along with obesity and hyper-insulinemia, are proposed for associating whole milk with fatal prostate cancer (31). Other suggested mechanisms include increasing levels of ionized calcium in the blood, as prostate cancer cells express both calcium-sensing receptors and calcium-dependent voltage-gated channels, the stimulation of which by extracellular calcium increases prostate cancer cell growth (44) (35). In African American men, positive associations between calcium and aggressive prostate cancer have been linked to single-nucleotide polymorphisms in the CDX-2 binding site of the VDR gene (32).
- Calcium may interfere with the absorption of iron, magnesium, and zinc (48) (49).
- High consumption of calcium has been associated with an increased risk of prostate cancer (30) (31) (32) (34) (49) (50) and milk-alkali syndrome (51).
- For those with chronic kidney disease, there is an increased risk of cardiovascular disease associated with calcium supplements (52).
- Calcium and vitamin D supplementation may increase the risk for kidney stones (42) (48) (53) (84). Increasing calcium via diet rather than supplements may lower the risk (54). For those who must take calcium in supplement form, adequate fluid intake is important to help reduce this risk (55).
Common: Constipation, flatulence, chalky taste and dry mouth; hypercalciuria and hypercalcemia in older women (58)
Excessive intake: Constipation, vascular and soft-tissue calcification, nephrolithiasis, hypercalciuria, hypercalcemia, increased risk for prostate cancer (48) (32) (34). Intake of high dose calcium supplements (>1 g/day) can increase the risk of ischemic stroke (76).
- Proton Pump Inhibitors: May significantly reduce calcium absorption (62).
- Cardiac glycosides: Calcium may increase risk of cardiac arrhythmia, although this is based on intravenous calcium, early case reports, and animal models (64).
- Quinolones: Calcium may reduce absorption of quinolones (65).
- Tamoxifen: Calcium may increase the risk of hypercalcemia (66).
- Tetracyclines: Calcium may reduce absorption of tetracyclines (67).
- Levothyroxine (to treat hypothyroidism): Taking calcium carbonate within 4 hours of this drug may decrease its absorption by nearly one-third (68).
- Estrogen therapy (for osteoporosis; positive interaction): Supplemental calcium and vitamin D may have additive benefits on bone health (28).
- Iron, zinc, or magnesium: Calcium can compete or interfere with their absorption (48) (49).
Dosage (OneMSK Only)
How It Works
Calcium plays several essential roles in the human body. It is a main structural element in bone, which acts as a reservoir for calcium so that stable blood calcium levels are maintained. This is necessary for 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 other processes, resulting in osteoporosis.
Inadequate blood calcium levels are linked to high blood pressure. Whether the use of calcium supplements can increase the risk for heart disease or cardiac events is still unclear. Adequate calcium intake may help prevent osteoporosis, bone fracture, and colorectal cancer. The links of calcium supplements to increased prostate cancer risk may occur for reasons other than the supplements themselves.
Calcium is available from a number of foods, and can be found in some antacids as well. Because the risks and benefits of calcium supplements are still unclear and calcium can interact with the absorption of a number of drugs, patients should check with their healthcare provider about whether supplementation is needed.
Purported Uses and Benefits
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. There does appear to be an increased risk for prostate cancer, but a decreased risk for colorectal cancer with calcium supplements. Additional studies are warranted.
To reduce cardiovascular disease risk
Studies are mixed on whether calcium supplementation protects against cardiovascular disease or increases risk for this disease. Additional studies are needed to clarify under what circumstances calcium may confer benefits or risks.
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
Some, but not all, large clinical trials have found that calcium supplementation prevents bone loss, especially in older women. Reduced bone loss may also prevent fractures.
Do Not Take If
- You have hypothyroidism, high blood calcium levels, or low blood phosphate levels: Consult your doctor before starting calcium supplements.
- You are taking cardiac glycosides such as digoxin: Calcium may increase the risk of irregular heartbeat.
- You are being treated with tamoxifen: Calcium may increase the risk of abnormally high levels of calcium in the blood. This can increase the risk of kidney stones.
- You are taking fluoroquinolone or tetracycline antibiotics: Calcium may reduce the absorption of these antibiotics into the bloodstream. Calcium should be taken 2 hours before or 1 hour after taking these medicines.
- You are taking drugs to treat an underactive thyroid: Taking calcium carbonate within 4 hours of this drug may decrease its absorption by almost 33%.
- You are taking iron, zinc, or magnesium: Calcium can interfere with their absorption.
- Constipation, flatulence, chalky taste, and dry mouth; higher calcium levels in blood or urine in older women
- Modest increased risk of urinary stones with long-term use
- Nausea occurs rarely
- Intake of high-dose calcium supplements (>1 g/day) can increase the risk of stroke
- Increasing dietary calcium from a variety of foods including non-dairy sources instead of taking supplements has been associated with a lower risk of kidney stones.
- For those requiring calcium supplementation, increasing fluid intake may reduce the risk of kidney stones.
- Some medications such as those for gastric reflux may reduce calcium absorption.
- Calcium may be an ingredient in some antacids.