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[Pharmacology](https://durgical.com/en/pharmacology/) · [Antacids](https://durgical.com/en/pharmacology/classes/antacids/)

# Magnesium hydroxide

An antacid that is also a saline laxative: it neutralises acid and draws water into the stool. Magnesium can build up when the kidneys fail.

Updated 2026-11-01

- Class: [Antacids](https://durgical.com/en/pharmacology/classes/antacids/)
- Target: Gastric acid (H⁺) in the stomach lumen; water in the bowel
- ATC code: A02AA04

Also called: Milk of magnesia

## Mechanism of action

In the stomach magnesium hydroxide neutralises hydrochloric acid, forming magnesium chloride and water; it is a stronger antacid than aluminium hydroxide. Magnesium ions are poorly absorbed, so in the bowel they hold water by osmosis, soften the stool and speed transit. A small fraction of magnesium is absorbed and is cleared by the kidney.

- Neutralises stomach acid into magnesium chloride and water
- Unabsorbed magnesium holds water in the bowel by osmosis
- Softer, bulkier stool speeds bowel movements
- The little magnesium absorbed is cleared by the kidney

## Pharmacokinetics

**Absorption**
Mostly not absorbed; a small fraction of magnesium enters the blood.

**Elimination**
Absorbed magnesium is excreted by the kidney.

**Onset and duration**
As a laxative, a bowel movement usually follows within 30 minutes to 6 hours.

## Indications

- Occasional constipation, short term, in children and adults. [Constipation and faecal impaction →](https://durgical.com/en/conditions/constipation-and-faecal-impaction/)
- Heartburn, acid indigestion and upset stomach, often combined with aluminium hydroxide. [Gastro-oesophageal reflux disease (GERD) →](https://durgical.com/en/conditions/gastro-oesophageal-reflux-disease/)

## Adverse effects

### Common

- Loose, watery or more frequent stools

### Serious (rare, important)

- Magnesium accumulation in kidney disease
- Blood in the stool, or no bowel movement within 6 hours of a laxative dose: the drug is stopped and the patient reviewed

## Contraindications

- Allergy to magnesium hydroxide or other ingredients of the product

## Key interactions

**Other oral medicines (for example tetracyclines, digoxin, iron)**
Lower absorption; they are taken at least two hours before or after.

## What is monitored

- Kidney function and magnesium in kidney disease
- Bowel habit, fluid intake and electrolytes with frequent use

## Administration and safety

- Suspension, tablets or chewable tablets by mouth; the suspension is shaken
- Children's products are used for children, never adult products
- Other medicines are separated by at least two hours
- As a laxative it is not used for more than a week without medical review

## Nursing care

- Check kidney function before regular use
- Record stool frequency and watch for dehydration with repeated doses
- Space other oral drugs at least two hours apart
- Teach the patient to report blood in the stool or no bowel movement within 6 hours

## Pregnancy and breastfeeding

The patient leaflet asks pregnant or breastfeeding women to discuss its use with their doctor.

## Memory aid

«MAGnesium MAGnetises water»: unabsorbed magnesium pulls water into the bowel, so it loosens the stool, and the failing kidney cannot clear what is absorbed.

## Test yourself

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

## How does magnesium hydroxide work as a laxative?

Poorly absorbed magnesium holds water in the bowel by osmosis, softening the stool and speeding transit.

## Which antacid causes diarrhoea, and which causes constipation?

Magnesium hydroxide causes diarrhoea; aluminium hydroxide causes constipation.

## Why are magnesium antacids used with caution in kidney disease?

A small amount of magnesium is absorbed and normally cleared by the kidney; in kidney failure it can accumulate.

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## Related drugs

- [Aluminium hydroxide](https://durgical.com/en/pharmacology/aluminium-hydroxide/)
- [Calcium carbonate](https://durgical.com/en/pharmacology/calcium-carbonate/)
- [Magnesium sulfate](https://durgical.com/en/pharmacology/magnesium-sulfate/)

## Related conditions

- [Constipation and faecal impaction](https://durgical.com/en/conditions/constipation-and-faecal-impaction/)
- [Gastro-oesophageal reflux disease (GERD)](https://durgical.com/en/conditions/gastro-oesophageal-reflux-disease/)

[← Back to the class: Antacids](https://durgical.com/en/pharmacology/classes/antacids/)

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

Sources: [Magnesium Hydroxide: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a601073.html) (MedlinePlus (U.S. National Library of Medicine), 2019); [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); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=A02AA) (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/pharmacology/magnesium-hydroxide/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/magnesium-hydroxide/