Calcium carbonate !
A fast-acting antacid for heartburn and a common calcium supplement. Absorbed calcium can cause alkalosis if it is overused.
Updated 2026-11-01
Mechanism of action
Calcium carbonate reacts with hydrochloric acid in the stomach to form calcium chloride, water and carbon dioxide, which raises gastric pH and lowers pepsin activity. It neutralises quickly and completely. Part of the calcium is absorbed, which makes it useful as a supplement when dietary calcium is low but can cause alkalosis with heavy use, so as an antacid it is meant for brief use.
- Carbonate reacts with stomach acid
- Calcium chloride, water and CO₂ form; gastric pH rises
- Pepsin works less and heartburn eases
- Part of the calcium is absorbed: supplement and alkalosis risk
Pharmacokinetics
- Absorption
- Part of the calcium is absorbed in the small intestine; the antacid effect is local.
- Onset and duration
- Rapid, complete neutralisation of stomach acid.
Indications
- Heartburn, acid indigestion and upset stomach (short-term relief). Gastro-oesophageal reflux disease (GERD) →
- Calcium supplement when dietary calcium is not enough. Hypocalcaemia →
Adverse effects
Common
- Constipation
- Belching (carbon dioxide)
- Stomach upset, vomiting and loss of appetite
- Dry mouth, metallic taste and increased urination
Contraindications
- Allergy to calcium carbonate
Key interactions
- Tetracyclines, iron and other oral medicines
- Calcium can lower their absorption; other medicines are separated by one to two hours.
- Digoxin
- Antacids can lower its absorption.
What is monitored
- Acid–base status and kidney function with heavy or long-term use
- Symptoms that persist beyond two weeks, which need medical review
Administration and safety
- Chewable tablets are chewed thoroughly and followed by a glass of water
- Other medicines are separated from it by one to two hours
- As an antacid it is not used for more than two weeks without medical review
Nursing care
- Check the medication list for drugs that calcium binds (antibiotics, iron, thyroid hormone) and plan the timing
- Ask about constipation and belching
- Teach the patient that heartburn lasting more than two weeks, trouble swallowing or weight loss need evaluation
- In kidney disease, check calcium before regular use
Pregnancy and breastfeeding
The patient leaflet asks pregnant or breastfeeding women to discuss its use with their doctor.
Test yourself
Think of the answer, then tap the question to see it.
Why does calcium carbonate cause belching?
Its reaction with stomach acid releases carbon dioxide gas.
Why is calcium carbonate called an absorbable antacid, and what is the risk?
Part of its calcium is absorbed; heavy use can cause metabolic alkalosis and high calcium.
How long apart from calcium carbonate are other medicines taken?
One to two hours, because calcium can lower their absorption.
Related drugs
Related conditions
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: Calcium Carbonate: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2024); Medications for the Treatment of Gastric Acidity - Gastroenterology - MSD Manual Professional Edition (MSD Manual Professional Edition, 2025); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026).
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Representative model, for educational purposes only.