Calcium carbonate (mineral salt)

Calcium carbonate

Calcium carbonate is a licensed over-the-counter antacid that relieves heartburn for a few hours, and it is safe for most people at label doses, but it does not treat the cause and the label limits use to two weeks. As a bone supplement the evidence is weak: the largest recent meta-analysis (69 trials) found little to no reduction in fractures, and whether calcium supplements raise heart attack risk is still argued over. It can cause constipation, and it can stop thyroid tablets and some antibiotics being absorbed, so it needs spacing from other medicines.

Written by

Healthy and Herbal editorial team

Reviewed by

Healthy and Herbal editors

Updated

30 September 2026

White chalky tablets in a small dish beside a glass of water

At a glance

Last reviewed 30 September 2026

Evidence

Limited

Best for

Short-term heartburn relief

Typical form & use

Chewable tablet, 1,000 mg as needed

Onset

Same night

Safety

Can block thyroid tablets and some antibiotics. 2 known interactions, most serious with antibiotics.

Check with your medications →

What it's used for

Evidence is rated per use. The same ingredient can be strong for one thing and traditional for another.

Use	Evidence	What the research shows
Heartburn and indigestion (antacid)	Limited	Calcium carbonate is a standard OTC antacid: the US label for a 1,000 mg chewable tablet lists relief of heartburn, acid indigestion, sour stomach and upset stomach, and the NHS says antacids quickly relieve symptoms for a few hours but do not treat the underlying cause. We rate this 2, not higher, because the sources we cite are a label and patient guidance rather than trials; the small acid-measurement studies we read were 24-person crossovers (see research notes).^6,7
Preventing fractures (supplement)	Limited	A 2026 BMJ systematic review of 69 randomised trials (153,902 people, mostly community-dwelling and not at high risk) found calcium alone gave a risk ratio for any fracture of 0.91 (95% CI 0.81 to 1.01; 11 trials, 9,067 people; moderate certainty), and calcium with vitamin D 0.91 (0.84 to 0.99; high certainty). The authors judged the absolute benefit too small to matter. The US Preventive Services Task Force (2018) recommends against 1,000 mg or less of calcium with 400 IU or less of vitamin D for fracture prevention in postmenopausal women living in the community (grade D), and found evidence insufficient for men, premenopausal women and higher doses. In the Women's Health Initiative, 1,000 mg calcium (as carbonate) plus vitamin D3 raised hip bone density by 1.06% but did not significantly reduce hip fracture (hazard ratio 0.88, 0.72 to 1.08). None of this speaks to people with a diagnosed deficiency or on osteoporosis drugs, where the trials did not look.^1,2,5
Heart attack and stroke risk (harm signal)	Limited	Not a use but an open question. A 2010 BMJ meta-analysis of calcium supplements without vitamin D (five trials with patient-level data, 8,151 people) reported more heart attacks, hazard ratio 1.31 (95% CI 1.02 to 1.67). A 2023 meta-analysis of 11 placebo-controlled trials found no significant excess: calcium alone relative risk 1.15 for heart attack (0.88 to 1.51; 8,634 people) and calcium with vitamin D 1.09 (0.95 to 1.25; 46,804 people), and the authors said it ruled out excess risks above about 0.3 to 0.5% a year. The two analyses disagree, and neither can settle it, so we rate the concern 2 (mixed).^3,4
Kidney stones (harm signal)	Limited	In the Women's Health Initiative randomised trial, 1,000 mg calcium as carbonate plus vitamin D3 raised the risk of kidney stones (hazard ratio 1.17, 95% CI 1.02 to 1.34) over about 7 years in 36,282 postmenopausal women. The USPSTF names kidney stones as the harm to weigh against any fracture benefit. This was a calcium and vitamin D combination, so we cannot say how much was the calcium alone.^5,2

How much

Ranges below are what studies used and what supplements commonly contain. They are not a recommendation for you. Start at the low end.

Antacid label dose (US)

2 to 3 tablets of 1,000 mg as symptoms occur⁶

For adults and children 12 and over. The label says no more than 7 tablets in 24 hours (5 if pregnant), not to use the maximum for more than 2 weeks except on medical advice, and to see a doctor if symptoms last more than 2 weeks. Each tablet has 410 mg elemental calcium.

Studied for bones

1,000 mg elemental calcium a day^5,2

The Women's Health Initiative gave 1,000 mg of elemental calcium as carbonate with 400 IU vitamin D3 daily for about 7 years. The USPSTF uses this figure as its threshold: 1,000 mg calcium or less with 400 IU vitamin D or less is not recommended for fracture prevention in postmenopausal women.

Elemental versus tablet weight

1,000 mg carbonate = about 410 mg calcium^6,5

Supplement labels may quote either figure, so check which one you are reading before comparing with trial doses, which are given as elemental calcium.

Timing · As an antacid, take it when symptoms occur or are expected: the NHS suggests with or soon after meals and just before bed. The NHS also says not to take other medicines within 2 to 4 hours of an antacid.With food? · The NHS says antacids are best with food or soon after eating because that is when heartburn is likely, and the effect may last longer.How long to trial · Relief from an antacid lasts a few hours, per the NHS. If you need one most days, or for more than 2 weeks, the label and the NHS both say to see a doctor or pharmacist rather than carry on.

How people take it

Chewable antacid tablet1,000 mg, 2 to 3 tablets as neededThe label says to chew or crush tablets completely before swallowing. Some brands combine calcium carbonate with magnesium carbonate (the NHS names Rennie).

Calcium supplement tabletCheck the elemental calcium per tabletSame compound sold as a supplement. The fracture trials used 1,000 mg elemental calcium a day, usually with vitamin D3.

Safety

Who should avoid

Levothyroxine users — calcium carbonate cut absorption in a 20-person study
Tetracycline or quinolone antibiotics — ask a pharmacist how to space doses
Iron tablets — calcium carbonate significantly reduces iron absorption
Past kidney stones — a large trial found more stones with calcium plus vitamin D
Kidney disease, or large regular doses — risk of hypercalcaemia and milk-alkali syndrome
Pregnancy — the US label caps antacid use at 5 tablets in 24 hours
Children under 12 — the label gives no dose; ask a doctor

Known interactions

CautionAntibiotics — Tetracyclines bind metal ions in antacids and their absorption can fall by more than 90%; the same review reports 50 to 90% lower absorption of ciprofloxacin and ofloxacin with aluminium and magnesium hydroxide antacids, and does not quantify it for calcium carbonate itself.

CautionThyroid medication — In 20 people on stable levothyroxine, taking 1,200 mg elemental calcium as carbonate with it for 3 months lowered free thyroxine and raised thyroid-stimulating hormone from 1.6 to 2.7 mIU/L, with 20% above the normal range, and levels recovered after stopping.

Questions people ask

What are the side effects of calcium carbonate?

The NHS lists constipation or diarrhoea, wind, stomach cramps and feeling sick as the common ones, which usually pass when you stop. Larger or long-term amounts carry rarer risks: kidney stones (hazard ratio 1.17 with calcium plus vitamin D in a large trial) and milk-alkali syndrome, a dangerous mix of high blood calcium, kidney failure and alkalosis from taking large amounts of calcium with absorbable alkali. It has been described more often since calcium carbonate became widely used for osteoporosis prevention.

What is calcium carbonate used for?

Mainly two things: as an antacid to relieve heartburn and indigestion for a few hours, and as a calcium supplement for bone health. The antacid use is long established, but limited by the label to 2 weeks of maximum dosing; the bone use has weak evidence, since the latest large meta-analysis found little to no effect on fractures.

Do calcium supplements prevent fractures, and do they harm the heart?

A 2026 BMJ meta-analysis of 69 trials found little to no reduction in fractures, and the USPSTF recommends against low-dose calcium with vitamin D for postmenopausal women in the community. On the heart, a 2010 meta-analysis found more heart attacks with calcium alone (hazard ratio 1.31), but a 2023 meta-analysis found no significant excess. Neither answer is settled.

Can I take calcium carbonate with my thyroid tablet or antibiotic?

Not at the same time, without asking. Calcium carbonate reduced levothyroxine absorption and raised thyroid-stimulating hormone in one study, and antacids can cut tetracycline absorption by more than 90%. The NHS says to keep other medicines 2 to 4 hours away from an antacid; ask a pharmacist about your specific medicine.

Sources

Last reviewed 30 September 2026 · Next review March 2027 · How we rate evidence