Calcium Carbonate Antacid

Calcium carbonate is a fast-acting, high-capacity antacid that neutralises gastric acid within minutes. Part of the calcium is absorbed, so regular high intake can cause hypercalcaemia and milk-alkali syndrome, particularly with kidney impairment or thiazide diuretics.

Classification. Antacid; calcium salt.

Mechanism. Reacts with hydrochloric acid to form calcium chloride, water and carbon dioxide, raising gastric pH. Calcium may stimulate gastrin release, contributing to acid rebound after repeated doses.

Pharmacodynamics. Onset within minutes; duration up to 3 hours when taken after a meal. Carbon dioxide release causes belching and bloating.

Pharmacokinetics. Calcium carbonate is 40% elemental calcium (1.25 g gives 500 mg calcium). About 20 to 30% of calcium is absorbed, more in the presence of vitamin D. Absorbed calcium is excreted by the kidneys; the remainder leaves in faeces.

Adult oral doses unless stated. Dose by milligrams of calcium carbonate and check the product strength, since elemental calcium content differs from the salt weight.

Heartburn and dyspepsia.

• 500 mg to 1.5 g calcium carbonate orally, chewed, when symptoms occur or 1 hour after meals and at bedtime.

• Maximum about 7.5 g calcium carbonate daily (3 g elemental calcium), for no more than 2 weeks without review.

• Products combined with magnesium carbonate or alginate are widely used.

Heartburn in pregnancy. 500 mg to 1 g calcium carbonate chewed as needed after meals and at bedtime, within the daily maximum. Total elemental calcium from all sources should stay under about 2.5 g daily.

Paediatric dyspepsia. Age 12 years and over: adult dosing. Under 12 years, use product-specific age dosing and only for short periods.

Calcium supplementation and phosphate binding with calcium carbonate use different doses and timing from antacid use and are not covered by these regimens.

Contraindications. Hypercalcaemia; hypercalciuria; calcium-containing kidney stones; severe kidney impairment for regular use.

Common adverse effects. Belching, bloating, flatulence, constipation and acid rebound after repeated doses.

Serious adverse effects.

• Milk-alkali syndrome: hypercalcaemia, metabolic alkalosis and acute kidney injury. It occurs with prolonged high intake, often several grams of calcium daily, and especially with thiazides, kidney impairment, vomiting or dehydration.

• Hypercalcaemia causing nausea, polyuria, confusion and nephrocalcinosis.

• Kidney stones with prolonged high intake.

Major interactions. Calcium chelates or adsorbs many drugs; separate dosing.

• Tetracyclines, fluoroquinolones and oral iron: give the other drug 2 hours before or 4 to 6 hours after.

• Integrase inhibitors (dolutegravir, raltegravir, bictegravir): give the antiretroviral 2 hours before or 6 hours after when fasting. Dolutegravir and bictegravir may be taken at the same time as calcium if taken with food.

• Levothyroxine and bisphosphonates: separate by at least 4 hours and 2 hours respectively.

• Digoxin: hypercalcaemia increases toxicity risk; avoid regular high doses.

• Thiazide diuretics: reduced calcium excretion raises the risk of hypercalcaemia and milk-alkali syndrome.

• Enteric-coated tablets may dissolve early.

Special groups.

• Pregnancy: widely used for heartburn and acceptable within the daily calcium limit. Avoid combining with high-dose calcium supplements without a calculated total.

• Breastfeeding: compatible.

• Children: short-term use only.

• Older adults: higher risk of constipation, hypercalcaemia and interaction; review regular use.

• Kidney impairment: avoid regular antacid use when eGFR is under 30 mL/min/1.73 m2 unless directed by nephrology; monitor calcium if used.

• Liver impairment: no dose change; avoid constipation that may precipitate hepatic encephalopathy.

Formulations. Chewable tablets (commonly 500 mg to 1.25 g calcium carbonate); oral suspension; combination products with magnesium carbonate or alginate.

Administration. Chew thoroughly before swallowing or take suspension well shaken. Take when symptoms occur or 1 hour after meals and at bedtime. Do not take with large volumes of milk or other calcium sources on a regular basis.

Baseline. Calcium and kidney function before regular use in kidney impairment, with thiazides or in older adults.

Monitoring.

• Serum calcium, bicarbonate and creatinine if used regularly at high dose or if nausea, confusion, polyuria or rising creatinine develop.

• Total daily calcium intake from all sources in pregnancy and in those on supplements.

Therapeutic drug monitoring. Not applicable.

Stopping. Stop if hypercalcaemia, alkalosis or rising creatinine develop. Management of milk-alkali syndrome is stopping calcium and alkali, IV sodium chloride 0.9% and correction of hypovolaemia; most cases recover kidney function. Antacid use for over 2 weeks needs assessment for an underlying cause.

Referral and escalation. Endoscopy for persistent dyspepsia or alarm features. Urgent admission for hypercalcaemia with confusion, AKI or ECG changes.