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
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 →
- Heartburn, acid indigestion and upset stomach, often combined with aluminium hydroxide. Gastro-oesophageal reflux disease (GERD) →
Adverse effects
Common
- Loose, watery or more frequent stools
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.
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.
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: Magnesium Hydroxide: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2019); 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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