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Moderate evidenceVitamins & minerals

Calcium Carbonate

The cheapest and most concentrated calcium salt (40% elemental calcium) and a fast antacid. It is absorbed and demonstrably acts on bone turnover, but supplementing calcium does very little for fracture risk.

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What the science says

  • Absorbed and biologically active on bone turnover Moderate evidence

    In a randomized trial in 100 postmenopausal women, 1 g/day of calcium as carbonate or citrate raised serum calcium for up to 8 hours after each dose and reduced bone resorption markers (CTX) both acutely and over 3 months.

  • Little to no fracture prevention Strong evidence

    A 2026 systematic review of 69 randomized trials (153,902 participants) found little to no effect of calcium supplements on any fracture (RR 0.91, 95% CI 0.81–1.01) or on falls, with moderate-to-high certainty. Combined calcium plus vitamin D reached at most a marginal reduction.

Dosage & safety

Studied dose500–600 mg elemental calcium per dose, taken with food because absorption depends on stomach acid. Total intake from food plus supplements of about 1000–1200 mg/day; more than ~500 mg at once is poorly absorbed.
SafetyConstipation, bloating and acid rebound are common. It repeatedly raises serum calcium through the day, which is the basis of the ongoing debate about cardiovascular risk from calcium supplements. It blocks absorption of levothyroxine, iron, and tetracycline and quinolone antibiotics — separate by at least four hours. People with kidney stones, hypercalcemia or on proton-pump inhibitors (which reduce carbonate absorption) should ask a doctor; getting calcium from food is preferable where possible.

How to take it

TimingSplit into doses of 500–600 mg; more than that at once is poorly absorbed regardless of when you take it.
With food?With food — carbonate depends on stomach acid to dissolve.
Worth knowingIf you take an acid-blocking drug, calcium citrate absorbs without stomach acid and is the sensible swap.

Interactions

With medications

  • levothyroxine

    Calcium binds it in the gut and can markedly reduce absorption. separate by at least 4 hours

  • tetracycline and quinolone antibiotics

    Calcium chelates the antibiotic and reduces its effect. separate by at least 4 hours

  • bisphosphonates (alendronate, risedronate)

    Calcium blocks their absorption almost completely. take the bisphosphonate first thing on an empty stomach, at least 30–60 minutes before any calcium

  • proton-pump inhibitors

    Carbonate needs stomach acid to dissolve, so absorption drops sharply. switch to calcium citrate, which does not need acid

  • thiazide diuretics

    They reduce calcium excretion and raise the risk of hypercalcemia. have calcium levels checked if using both long term

With other supplements

  • iron

    Calcium reduces non-heme iron absorption. separate by at least 2 hours

  • zinc

    High calcium doses compete with zinc for absorption. take at different times of day

Well-established interactions only — this is not a complete list. Always tell your doctor and pharmacist what you take.

Frequently asked questions

Do calcium carbonate supplements prevent fractures?

Barely: a 2026 review of 69 randomized trials in 153,902 people found little to no effect of calcium supplements on fractures or falls, a strong-grade finding. Calcium carbonate is absorbed and acts on bone turnover, but that does not translate into fewer fractures.

How much calcium carbonate should I take at once?

Take 500–600 mg of elemental calcium per dose, since more than about 500 mg at once is poorly absorbed. Aim for about 1000–1200 mg/day total from food plus supplements; calcium carbonate is 40% elemental calcium.

Should calcium carbonate be taken with food?

Yes: calcium carbonate depends on stomach acid to dissolve, so take it with food. If you take a proton-pump inhibitor, switch to calcium citrate, which absorbs without stomach acid.

What medications should not be taken with calcium carbonate?

Calcium carbonate blocks absorption of levothyroxine and tetracycline and quinolone antibiotics, so separate them by at least 4 hours. Take bisphosphonates first thing on an empty stomach, at least 30–60 minutes before any calcium.

Scientific references

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