Milk of magnesia is a trusted over-the-counter medicine containing magnesium hydroxide, widely used in the UK for short-term relief of constipation and indigestion. Available without prescription from pharmacies and supermarkets, this white suspension works through two distinct mechanisms: as an osmotic laxative that draws water into the bowel to soften stools, and as an antacid that neutralises stomach acid. Regulated by the MHRA, milk of magnesia has been a household remedy for over a century. Understanding how magnesium works in this preparation, along with proper dosing and safety precautions, ensures effective symptom relief whilst minimising risks. This article explores the clinical uses, administration guidance, and important considerations for safe use of milk of magnesia.

What Is Milk of Magnesia and How Does Magnesium Work?

Milk of magnesia is a widely used over-the-counter medicine containing magnesium hydroxide as its active ingredient. This white, chalky suspension has been a trusted remedy for digestive complaints for over a century. In the UK, it is available without prescription from pharmacies and supermarkets, with some products being Pharmacy-only (P) medicines and others available as General Sales List (GSL) items. All are regulated by the Medicines and Healthcare products Regulatory Agency (MHRA) for short-term relief of constipation and indigestion.

The mechanism of action of magnesium hydroxide depends on its intended use. As a laxative, it works through osmotic action in the bowel. When taken orally, magnesium hydroxide is poorly absorbed from the gastrointestinal tract. It draws water into the intestinal lumen by osmosis, which softens the stool and increases its volume. This stimulates peristalsis (the natural muscular contractions of the bowel), typically producing a bowel movement within 30 minutes to 6 hours. The osmotic effect makes it particularly useful for occasional constipation.

As an antacid, magnesium hydroxide neutralises excess stomach acid through a simple chemical reaction. When it encounters hydrochloric acid in the stomach, it forms magnesium chloride and water, raising the gastric pH and providing relief from heartburn and indigestion. This neutralising capacity is relatively rapid, offering symptomatic relief within minutes of administration.

The dual action of milk of magnesia makes it a versatile preparation, though the dose differs significantly between its laxative and antacid applications. Understanding these distinct mechanisms helps patients use the medicine appropriately for their specific symptoms whilst minimising the risk of adverse effects from incorrect dosing.

Uses and Benefits of Milk of Magnesia

Milk of magnesia is primarily indicated for two distinct clinical purposes, each with evidence-based benefits when used appropriately.

Relief of occasional constipation is the most common use. The osmotic laxative effect provides predictable relief for adults and children over three years of age experiencing infrequent bowel movements. Unlike stimulant laxatives, magnesium hydroxide is generally considered gentler on the bowel and does not typically cause cramping in most users. It is particularly useful for situational constipation caused by dietary changes, travel, or temporary immobility. For adults, bulk-forming laxatives are often recommended as first-line treatment, with osmotic laxatives like milk of magnesia added if needed. For children, macrogols are the first-line treatment for constipation according to NICE guidance (CG49). Lifestyle measures such as increased fibre and fluid intake should always be attempted first.

Short-term relief of indigestion and heartburn represents the second major indication. The antacid properties neutralise gastric acid, providing symptomatic relief from dyspepsia and stomach discomfort. This can be helpful after consuming rich or spicy foods, or for occasional symptoms. However, frequent or persistent heartburn or acid reflux requires proper medical assessment rather than ongoing self-treatment with antacids.

Some people also use milk of magnesia topically for skin conditions, though this is not a licensed use in the UK. There is limited evidence for dermatological applications, and topical use may cause skin irritation in some individuals. Patients should consult a healthcare professional before using it in this manner.

The benefits of milk of magnesia include its rapid onset of action, dual functionality, long safety record, and affordability. However, it is intended only for short-term, occasional use. NICE guidance emphasises that chronic constipation or persistent indigestion requires proper medical assessment to identify underlying causes and establish appropriate long-term management strategies rather than continued self-medication.

How to Take Milk of Magnesia Safely

Correct dosing and administration are essential to ensure both efficacy and safety when using milk of magnesia. The dose varies significantly depending on whether it is being used as a laxative or an antacid, and patients must follow the instructions on the product label or as directed by their pharmacist or GP.

For constipation (laxative dose):

• Adults and children over 12 years: typically 30–45 ml as a single daily dose, preferably taken at bedtime with a full glass of water

• Children aged 3–12 years: 5–10 ml daily, though paediatric use should be discussed with a healthcare professional first, as macrogols are the first-line treatment for children with constipation

• The laxative effect usually occurs within 30 minutes to 6 hours

For indigestion (antacid dose):

• Adults: typically 5–10 ml up to four times daily, taken between meals and at bedtime

• Children: antacid use in children should only be under medical supervision

• Shake the bottle thoroughly before each use to ensure the suspension is properly mixed

Important administration guidance:

• Always measure doses accurately using the provided measuring cup or spoon—never estimate

• Drink plenty of water throughout the day, particularly when using milk of magnesia as a laxative

• Take the medicine at least 2–4 hours before or after other medications, as magnesium can interfere with the absorption of certain drugs including antibiotics (tetracyclines, quinolones), bisphosphonates, levothyroxine, iron supplements, penicillamine, and some cardiac medications

• Some medicines may require longer separation times—check with your pharmacist

• Do not exceed the recommended dose or duration of use

Duration of treatment: Milk of magnesia should not be used for more than three days for constipation or two weeks for indigestion without consulting a doctor. Prolonged use can lead to electrolyte imbalances and may mask underlying conditions requiring medical attention. If symptoms persist or worsen, patients should seek professional advice rather than continuing self-treatment.

Side Effects and Precautions

Whilst milk of magnesia is generally well tolerated when used as directed, patients should be aware of potential adverse effects and important safety precautions.

Common side effects include:

• Diarrhoea: the most frequent adverse effect, particularly if the laxative dose is too high; this usually resolves when the dose is reduced or stopped

• Abdominal cramping or discomfort: mild cramping may occur as the bowel responds to the osmotic effect

• Nausea: some people experience mild nausea, especially with higher doses

• Chalky taste: the suspension has a characteristic taste that some find unpleasant

Serious adverse effects are uncommon but require immediate medical attention:

• Severe or persistent diarrhoea leading to dehydration

• Electrolyte disturbances: prolonged use or excessive doses can cause magnesium toxicity (hypermagnesaemia), presenting as muscle weakness, confusion, irregular heartbeat, or difficulty breathing

• Allergic reactions: though rare, symptoms may include rash, itching, swelling, or difficulty breathing

Important precautions and contraindications: Patients should not use milk of magnesia if they have:

• Moderate to severe kidney impairment (magnesium is excreted by the kidneys, and accumulation can occur)

• Symptoms of appendicitis or bowel obstruction (abdominal pain, nausea, vomiting)

• Acute abdominal symptoms including severe pain, vomiting, or fever

• Known allergy to magnesium hydroxide

Special populations requiring caution:

• Elderly patients: increased risk of dehydration, electrolyte imbalance and hypermagnesaemia; lower doses may be appropriate

• Pregnant and breastfeeding women: generally considered safe for occasional use, but should consult a healthcare professional first

• Children under 3 years: should not be given milk of magnesia without medical supervision

• Patients with renal impairment: at risk of hypermagnesaemia which can affect cardiac rhythm and cause hypotension

When to contact your GP: Seek medical advice if you experience:

• Constipation that persists beyond 3 days of treatment

• Persistent indigestion lasting more than two weeks

• Blood in stools or black, tarry stools

• Unexplained weight loss accompanying digestive symptoms

• Severe or worsening abdominal pain

• Symptoms of dehydration (excessive thirst, reduced urination, dizziness)

Patients taking regular medications should consult their pharmacist or GP before using milk of magnesia, as it can interact with numerous drugs by affecting their absorption or altering electrolyte balance. Avoid using other magnesium-containing products concurrently unless advised by a healthcare professional.

If you experience any suspected side effects, report them to the MHRA through the Yellow Card Scheme (yellowcard.mhra.gov.uk).

Frequently Asked Questions