An osmotic laxative (milk of magnesia)

Magnesium hydroxide

A gentle osmotic laxative that also works as an antacid, best known as milk of magnesia.

What is Magnesium hydroxide?

Magnesium hydroxide is an osmotic laxative that draws water into the bowel to soften the stool and relieve constipation, and it also neutralises stomach acid as an antacid. It is best taken with plenty of water and used short term. It needs caution in people with kidney problems because magnesium can build up.

Education and reference only. This is a plain-language guide to Magnesium hydroxide — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.

Written by Meds Global Health Clinical Team · Clinically reviewed by Dr Muhammad Suffyan, MB BS, Founder & Clinical Director · Last reviewed · How we review
Class: Laxatives → Brands: Milk of Magnesia
Magnesium hydroxide (Laxatives) — Meds Global Health reference card
Magnesium hydroxide — Laxatives.

What it is

Magnesium hydroxide is a simple, long-established medicine best known by the name milk of magnesia. It has two uses: as an osmotic laxative for short-term constipation, and as an antacid to relieve indigestion and heartburn. It is gentle and acts within a few hours when used for constipation.

How it works

As a laxative, magnesium hydroxide is poorly absorbed and stays in the bowel, drawing water in by osmosis. The extra water softens and bulks the stool and stimulates the bowel to move, producing a softer, easier motion. As an antacid, it reacts with and neutralises excess stomach acid to ease indigestion and heartburn.

Company & origin

Originated / developed by: Available as a generic medicine and as branded milk of magnesia..

A long-established remedy, marketed as milk of magnesia since the late nineteenth century.

Practical use

How to take Magnesium hydroxide

General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.

  • Shake the liquid well before use and take it with a full glass of water.
  • Drink plenty of fluids through the day while you are using it.
  • Use it for short-term relief rather than every day, unless advised otherwise.
  • Separate it in time from some other medicines, as it can reduce their absorption.
  • If constipation does not improve or keeps coming back, get it reviewed rather than continuing indefinitely.

Weighing it up

Advantages & disadvantages of Magnesium hydroxide

Advantages

  • Gentle and predictable relief of short-term constipation.
  • Also works as an antacid for indigestion and heartburn.
  • Inexpensive, familiar and available without prescription.

Disadvantages

  • Can cause loose stools, diarrhoea or stomach cramps if too much is taken.
  • Magnesium can build up in people with kidney problems, so it needs caution there.
  • Can reduce the absorption of some other medicines unless doses are spaced apart.

Practical use

Good to know

Drink plenty of water while taking it, as it works by pulling fluid into the bowel. It is meant for short-term use; ongoing constipation should be reviewed. Too much can cause loose stools or diarrhoea, and it should be used cautiously if you have kidney problems because magnesium can build up. Leave a gap between magnesium hydroxide and certain other medicines, as it can affect how well they are absorbed.

Who should not take it / use with caution

  • People with significant kidney problems, because magnesium can accumulate.
  • People with a suspected bowel blockage or severe, undiagnosed abdominal pain.
  • Used with caution in frail or older people and where fluid balance is a concern.

Monitoring

  • No routine tests for short-term use.
  • Kidney function may be checked in people with kidney problems or on long-term use.
  • Review if constipation persists or keeps returning.

Side effects

  • Loose stools or diarrhoea, especially if too much is taken.
  • Stomach cramps or wind.
  • Rarely, a build-up of magnesium causing weakness or drowsiness in people with poor kidney function.

Key interactions

  • Some antibiotics (such as tetracyclines and quinolones) — reduced absorption, so doses should be spaced apart.
  • Certain other medicines including some heart and thyroid medicines — absorption can be affected.
  • Other antacids or magnesium-containing products — added magnesium load.

Available as: oral liquid (suspension) and tablets

Answers

Magnesium hydroxide: frequently asked questions

How quickly does milk of magnesia work?

As a laxative it usually works within a few hours, so it is often taken when constipation needs gentle relief.

Why should I drink plenty of water with it?

It works by drawing water into the bowel, so good fluid intake helps it work properly and prevents dehydration.

Can I use it if I have kidney problems?

It should be used with caution, because magnesium can build up in the body when the kidneys are not working well. Check with your pharmacist or doctor.

Can I take it with my other medicines?

It can reduce the absorption of some medicines, including certain antibiotics, so it is best to leave a gap between them. Ask your pharmacist about timing.

Is it also an antacid?

Yes. Magnesium hydroxide neutralises stomach acid, so it is used both for constipation and to relieve indigestion and heartburn.

The wider class

About Laxatives

Magnesium hydroxide belongs to the laxatives class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.

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Authoritative sources

  • BNF
  • NICE CKS

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