What the evidence says. Several controlled trials; effects real but modest or context-dependent.
What is Calcium?
Calcium is a mineral used for corrects dietary deficiency and, with vitamin d, treats/prevents rickets and osteomalacia. NutriDex grades the human evidence as Moderate. Calcium is the most abundant mineral in the body; 99% sits in bone as hydroxyapatite while the tightly regulated remainder drives nerve conduction, muscle contraction (including the heartbeat), vascular tone, and blood clotting. Chronic dietary shortfall accelerates bone loss and, with vitamin D deficiency, causes rickets/osteomalacia and contributes to osteoporotic fracture. In well-nourished, community-dwelling adults, however, supplementation evidence is underwhelming: large meta-analyses (including the WHI) show no reduction in hip or total fractures, while signals of harm — modestly increased myocardial infarction risk in some pooled analyses, and a clear ~17% rise in kidney stones in the WHI — have made routine supplementation controversial. Benefit is most credible when correcting genuine deficiency or paired with vitamin D in institutionalized/vitamin-D-deplete older adults.
Purported Benefits
✓Corrects dietary deficiency and, with vitamin D, treats/prevents rickets and osteomalacia
✓Slows bone mineral density loss and modestly reduces fracture risk in deficient or institutionalized vitamin-D-replete older adults (calcium + vitamin D)
✓Supports normal neuromuscular function, cardiac rhythm, and blood coagulation (physiological role, not a supplement effect in replete people)
✓Modestly lowers blood pressure (~1-2 mmHg) and may reduce pre-eclampsia risk in low-intake pregnant women (WHO recommends supplementation where intake is low)
✓Reduced recurrent colorectal adenomas in one classic RCT, though a later larger trial was null — not an established benefit
✓Does NOT reduce fracture in healthy community-dwelling adults and is not recommended for that purpose by the USPSTF
Evidence by outcome
The same supplement can be well-proven for one use and unproven for another — here is the human evidence graded outcome by outcome.
Adult RDA 1000 mg/day (1200 mg for women 51+ and all adults 71+); typical supplement dose 500-600 mg elemental Ca taken once or split (absorption is best in doses <=500 mg). Tolerable Upper Intake Level (UL): 2500 mg/day for adults 19-50, 2000 mg/day for adults 51+. Aim to reach the target mainly through diet; supplement only the gap.
Active Compounds
Calcium carbonate (40% elemental Ca; cheapest, take with food for acid-dependent absorption)Calcium citrate (21% elemental Ca; absorbed without food, better for low-acid/PPI users)Other salts: lactate, gluconate, phosphateDietary sources: dairy (milk, yogurt, cheese), fortified plant milks and juices, canned sardines/salmon with bones, tofu set with calcium, leafy greens (kale, bok choy), almonds
Safety & Cautions
⚠
UL is 2000-2500 mg/day; excess causes constipation, hypercalcemia, and impaired absorption of iron, zinc, and magnesium. Supplemental (not dietary) calcium raised kidney stone incidence ~17% in the WHI RCT. Pooled analyses by Bolland et al. suggest supplemental calcium (especially without vitamin D) modestly increases myocardial infarction risk (~RR 1.2-1.27); the signal is debated and not seen consistently, but favors getting calcium from food. Key interactions: reduces absorption of levothyroxine, bisphosphonates, tetracycline and fluoroquinolone antibiotics, and iron (separate dosing by 2-4 h); thiazide diuretics plus high calcium can cause hypercalcemia; proton-pump inhibitors reduce carbonate absorption (prefer citrate). Avoid high-dose supplements in those with hypercalcemia, hyperparathyroidism, or a history of calcium kidney stones. Educational only — always check with your doctor or pharmacist before combining Calcium with any medicine.
Calcium drug interactions
Known or theoretical interactions between Calcium and common medications — educational, not exhaustive. Always check with your doctor or pharmacist before combining Calcium with any medicine.
⚠Caution
Levothyroxine & some antibiotics/bisphosphonates
Calcium binds these drugs in the gut and lowers their absorption; separate doses by 4+ hours.
Divalent calcium forms insoluble complexes with levothyroxine, quinolones, tetracyclines, bisphosphonates. NIH ODS — Calcium
Calcium is most often taken for Corrects dietary deficiency and, with vitamin D, treats/prevents rickets and osteomalacia, Slows bone mineral density loss and modestly reduces fracture risk in deficient or institutionalized vitamin-D-replete older adults (calcium + vitamin D), Supports normal neuromuscular function, cardiac rhythm, and blood coagulation (physiological role, not a supplement effect in replete people), Modestly lowers blood pressure (~1-2 mmHg) and may reduce pre-eclampsia risk in low-intake pregnant women (WHO recommends supplementation where intake is low). The skeletal mineral with a supplement story that's more cautionary than it looks
Does Calcium work — what does the evidence say?
Moderate evidence. Several controlled trials; effects real but modest or context-dependent. Calcium is the most abundant mineral in the body; 99% sits in bone as hydroxyapatite while the tightly regulated remainder drives nerve conduction, muscle contraction (including the heartbeat), vascular tone, and blood clotting. Chronic dietary shortfall accelerates bone loss and, with vitamin D deficiency, causes rickets/osteomalacia and contributes to osteoporotic fracture. In well-nourished, community-dwelling adults, however, supplementation evidence is underwhelming: large meta-analyses (including the WHI) show no reduction in hip or total fractures, while signals of harm — modestly increased myocardial infarction risk in some pooled analyses, and a clear ~17% rise in kidney stones in the WHI — have made routine supplementation controversial. Benefit is most credible when correcting genuine deficiency or paired with vitamin D in institutionalized/vitamin-D-deplete older adults.
What is the typical dose of Calcium?
Adult RDA 1000 mg/day (1200 mg for women 51+ and all adults 71+); typical supplement dose 500-600 mg elemental Ca taken once or split (absorption is best in doses <=500 mg). Tolerable Upper Intake Level (UL): 2500 mg/day for adults 19-50, 2000 mg/day for adults 51+. Aim to reach the target mainly through diet; supplement only the gap.
Is Calcium safe? Any cautions or side effects?
UL is 2000-2500 mg/day; excess causes constipation, hypercalcemia, and impaired absorption of iron, zinc, and magnesium. Supplemental (not dietary) calcium raised kidney stone incidence ~17% in the WHI RCT. Pooled analyses by Bolland et al. suggest supplemental calcium (especially without vitamin D) modestly increases myocardial infarction risk (~RR 1.2-1.27); the signal is debated and not seen consistently, but favors getting calcium from food. Key interactions: reduces absorption of levothyroxine, bisphosphonates, tetracycline and fluoroquinolone antibiotics, and iron (separate dosing by 2-4 h); thiazide diuretics plus high calcium can cause hypercalcemia; proton-pump inhibitors reduce carbonate absorption (prefer citrate). Avoid high-dose supplements in those with hypercalcemia, hyperparathyroidism, or a history of calcium kidney stones.
How many studies support Calcium?
NutriDex cites 9 sources for Calcium, graded "Moderate".
Does Calcium interact with any medications?
Yes — known or theoretical interactions include: Thyroid medication (levothyroxine) (caution). This is educational and not exhaustive; always check with your doctor or pharmacist before combining Calcium with any medicine.
Cite this page
APA
Peh, D. (2026). Calcium: Benefits, Dosage, Side Effects & Evidence. NutriDex — The Supplement Research Compendium. Retrieved 16 Sept 2026, from https://nutridex.info/s/calcium
BibTeX
@misc{nutridex_calcium,
author = {Peh, Daryl},
title = {Calcium: Benefits, Dosage, Side Effects \& Evidence},
year = {2026},
howpublished = {NutriDex --- The Supplement Research Compendium},
url = {https://nutridex.info/s/calcium},
note = {Reviewed by Dr Daryl Peh, MBBS Singapore, MMed FM. Accessed 2026-09-16}
}
For medical claims, citing the underlying primary studies linked above is preferred. NutriDex is an educational reference, not medical advice.