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[Pharmacology](https://durgical.com/en/pharmacology/) · Class

# Antacids

Bases that neutralise acid already in the stomach. They relieve heartburn quickly but have been replaced by acid blockers for healing ulcers.

Updated 2026-11-01

## What they are

Calcium carbonate, aluminium hydroxide and magnesium hydroxide (alone or combined) relieve heartburn, acid indigestion and upset stomach. Acid-suppressing drugs have replaced them for healing ulcers; today they are used for short-term symptom relief.

## Mechanism of action

Antacids are weak bases that react with hydrochloric acid in the stomach lumen and raise gastric pH. Pepsin is less active as pH rises, and some antacids also adsorb pepsin. Absorbable antacids (calcium carbonate, sodium bicarbonate) neutralise fast and completely but can cause alkalosis; non-absorbable ones (aluminium and magnesium hydroxide) have fewer systemic effects. Aluminium slows the bowel and magnesium speeds it, so they are often combined.

Target: Gastric acid (H⁺) in the stomach lumen

## Key points

- Aluminium constipates and magnesium loosens the stool («aluminium = constipation, magnesium = diarrhoea»); combinations balance them.
- Antacids reduce the absorption of tetracyclines, digoxin, iron and many other drugs; other medicines are separated from them by a couple of hours.
- Magnesium and aluminium build up in kidney failure, so these antacids are used with caution there.
- Long-term aluminium hydroxide binds dietary phosphate and can cause phosphate depletion.
- Calcium carbonate is also a calcium supplement; absorbable antacids can cause alkalosis and are meant for brief use.

## Drugs in this class

- [**Calcium carbonate** A fast-acting antacid for heartburn and a common calcium supplement. Absorbed calcium can cause alkalosis if it is… Gastric acid (H⁺) in the stomach lumen](https://durgical.com/en/pharmacology/calcium-carbonate/)
- [**Aluminium hydroxide** A non-absorbed antacid that neutralises stomach acid and causes constipation. It binds phosphate, and its aluminium… Gastric acid (H⁺) in the stomach lumen](https://durgical.com/en/pharmacology/aluminium-hydroxide/)
- [**Magnesium hydroxide** An antacid that is also a saline laxative: it neutralises acid and draws water into the stool. Magnesium can build up… Gastric acid (H⁺) in the stomach lumen; water in the bowel](https://durgical.com/en/pharmacology/magnesium-hydroxide/)

## Others in the class

- Sodium hydrogen carbonate (bicarbonate)
- Magnesium trisilicate
- Simethicone (an antifoaming agent added to some antacids)

## Test yourself

Think of the answer, then tap the question to see it.

## Why are aluminium and magnesium hydroxide often combined in one antacid?

Aluminium tends to cause constipation and magnesium diarrhoea; together their bowel effects balance.

## Which antacid can cause low phosphate with long-term use, and why?

Aluminium hydroxide, because aluminium binds phosphate in the gut and prevents its absorption.

## Why is a tetracycline not taken at the same time as an antacid?

The calcium, magnesium or aluminium binds the antibiotic in the gut and lowers its absorption.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [Medications for the Treatment of Gastric Acidity - Gastroenterology - MSD Manual Professional Edition](https://www.msdmanuals.com/professional/gastrointestinal-disorders/gastritis-and-peptic-ulcer-disease/drug-treatment-of-gastric-acidity) (MSD Manual Professional Edition, 2025); [Aluminum Hydroxide and Magnesium Hydroxide: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a601013.html) (MedlinePlus (U.S. National Library of Medicine), 2023); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=A02A) (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

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