Milk of magnesia is a widely used over-the-counter medicine containing magnesium hydroxide, available without prescription across UK pharmacies and supermarkets. This white, chalky suspension has served as a trusted remedy for digestive complaints for over a century, working both as a laxative to relieve occasional constipation and as an antacid to neutralise stomach acid. Understanding the magnesium content, proper dosing, and safety considerations ensures effective use whilst minimising risks. This article explores how milk of magnesia works, its appropriate uses, and when professional medical advice should be sought.

What Is Milk of Magnesia and How Does It 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, available in the UK without prescription from pharmacies and supermarkets. The preparation typically contains magnesium hydroxide (usually labelled as 415 mg per 5 ml), giving it a milky appearance that inspired its common name.

The medicine works through two distinct mechanisms depending on the dose used. As a laxative, magnesium hydroxide draws water into the intestinal lumen through osmotic action. This increased fluid content softens the stool and stimulates peristalsis (the wave-like muscle contractions that move contents through the bowel), typically producing a bowel movement within 2–6 hours. As an antacid, lower doses neutralise excess stomach acid by reacting with hydrochloric acid to form magnesium chloride and water, providing relief from heartburn and indigestion.

The magnesium hydroxide in milk of magnesia is poorly absorbed from the gastrointestinal tract, with approximately 15% of the magnesium being absorbed. This limited systemic absorption means the medicine primarily acts locally within the digestive system, contributing to its safety profile for short-term use. However, even this limited absorption can be significant in people with impaired kidney function who cannot efficiently eliminate magnesium.

Milk of magnesia is classified as a saline laxative and belongs to the antacid class of medicines. It is regulated by the Medicines and Healthcare products Regulatory Agency (MHRA) in the UK and is considered appropriate for self-care management of occasional constipation and minor digestive discomfort in most adults and children over three years of age. It is not recommended for children under 3 years unless advised by a healthcare professional.

Magnesium Content in Milk of Magnesia

Understanding the magnesium content in milk of magnesia is important for safe use, particularly for individuals monitoring their magnesium intake or those with kidney problems. Standard UK formulations typically contain magnesium hydroxide 415 mg per 5 ml, though this varies slightly between brands. When used as a laxative (typically 30 ml for adults), this provides a substantial amount of magnesium hydroxide, though only a small proportion (around 15%) is actually absorbed into the bloodstream.

Magnesium is an essential mineral involved in over 300 enzymatic reactions in the body, including muscle and nerve function, blood glucose control, and bone health. The recommended daily allowance for magnesium in the UK is approximately 300 mg for men and 270 mg for women. While a laxative dose of milk of magnesia contains significant magnesium hydroxide, most passes through the digestive system without being absorbed.

When used occasionally as directed, the magnesium content is generally well tolerated. However, regular or excessive use can lead to magnesium accumulation, particularly in people with reduced kidney function who cannot efficiently excrete excess magnesium. Symptoms of hypermagnesaemia (elevated blood magnesium) include nausea, muscle weakness, low blood pressure, and in severe cases, cardiac arrhythmias.

It is worth noting that different formulations exist with varying concentrations. Some products marketed for antacid use contain lower amounts per dose (typically 5–10 ml servings with proportionally less magnesium), whilst laxative formulations use higher volumes. Always check the product label for specific magnesium content and follow the recommended dosing instructions. People taking magnesium supplements or medications containing magnesium should inform their pharmacist before using milk of magnesia to avoid excessive total intake.

Uses and Benefits of Milk of Magnesia

Milk of magnesia serves several therapeutic purposes, making it a versatile addition to the home medicine cabinet. Its primary uses include relief of occasional constipation and treatment of heartburn and indigestion.

For constipation, milk of magnesia provides relatively rapid relief compared to bulk-forming laxatives. It is particularly useful for short-term management of constipation caused by dietary changes, travel, or temporary immobility. The osmotic action helps restore normal bowel function without causing dependence. NICE guidance on constipation management recognises osmotic laxatives as appropriate first-line options in certain circumstances, though macrogol-based products are often preferred for longer-term use.

As an antacid, lower doses effectively neutralise stomach acid, providing symptomatic relief from heartburn, acid indigestion, and upset stomach. The alkaline magnesium hydroxide rapidly reacts with gastric acid, offering quick relief that typically lasts 30–60 minutes on an empty stomach and potentially longer after meals. This makes it suitable for occasional use when dietary indiscretion or stress triggers acid-related symptoms.

Beyond these licensed uses, some people use milk of magnesia topically for skin conditions. However, this topical application is not recommended due to insufficient evidence and potential for skin irritation. It remains an off-label use not supported by regulatory approval.

The benefits of milk of magnesia include its rapid onset of action, dual-purpose formulation, and long history of safe use when taken as directed. It is generally well tolerated, does not cause dependency, and is available at low cost. The medicine can be particularly helpful for people who need occasional relief but do not require daily laxative therapy. However, it should not replace addressing underlying causes of chronic constipation or recurrent indigestion through dietary modification, lifestyle changes, or medical investigation as appropriate.

Dosage and How to Take Milk of Magnesia Safely

Correct dosing is essential for safe and effective use of milk of magnesia. Dosage varies depending on whether the medicine is being used as a laxative or antacid, and differs by age group.

For constipation in adults and children over 12 years: The typical dose is 30–45 ml taken as a single daily dose, preferably at bedtime, or as directed on the product label. Some formulations may recommend slightly different amounts, so always follow the specific product instructions.

For children aged 3–12 years: Doses are weight and age-dependent, typically ranging from 5–15 ml. Consult the product information leaflet or speak with a pharmacist for age-appropriate dosing. Milk of magnesia is not recommended for children under 3 years except under medical supervision.

For antacid use: Lower doses of 5–10 ml are taken up to four times daily, usually after meals and at bedtime. The maximum daily dose should not exceed that specified on the product packaging.

Important administration guidance:

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

• Take with a full glass of water or plenty of water to aid effectiveness and prevent dehydration

• For laxative use, do not take for more than 3 consecutive days without medical advice

• Take at least 2 hours apart from most medications, and 4 hours apart from tetracyclines, quinolones, levothyroxine and certain antivirals, as milk of magnesia can interfere with absorption

• For antacid use, doses are typically taken after meals and at bedtime

Milk of magnesia should be stored at room temperature, away from heat and moisture. Do not freeze the suspension. Check the expiry date before use and discard any outdated product. If symptoms persist despite appropriate use, or if you find yourself needing the medicine regularly, consult your GP to investigate underlying causes rather than continuing self-treatment indefinitely.

Side Effects and Precautions

Whilst milk of magnesia is generally well tolerated when used as directed, it can cause side effects and is not suitable for everyone. Understanding potential adverse effects and contraindications helps ensure safe use.

Common side effects include:

• Diarrhoea – the most frequent adverse effect, particularly if the dose is too high or if individual sensitivity exists

• Abdominal cramping or discomfort – mild cramping may occur as the bowel responds to increased fluid

• Nausea – some people experience mild queasiness, especially with higher doses

• Chalky taste – the suspension has a distinctive flavour that some find unpleasant

These effects are usually mild and resolve once treatment is stopped. If diarrhoea becomes severe or persistent, discontinue use and seek medical advice, as excessive fluid loss can lead to dehydration and electrolyte imbalance.

Serious side effects are uncommon but require immediate medical attention:

• Signs of hypermagnesaemia (magnesium toxicity): severe muscle weakness, confusion, difficulty breathing, irregular heartbeat, or extreme drowsiness

• Severe allergic reactions: rash, itching, swelling (particularly of face, tongue, or throat), severe dizziness, or difficulty breathing

• Rectal bleeding or failure to have a bowel movement after use

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

Precautions and contraindications:

Milk of magnesia should be avoided in:

• Severe renal impairment – reduced ability to excrete magnesium increases toxicity risk

• Symptoms of appendicitis (severe abdominal pain, nausea, vomiting) – laxative use could worsen the condition

• Intestinal obstruction or undiagnosed abdominal pain

• Magnesium-restricted diets or known magnesium sensitivity

Use with caution and seek medical advice if you have:

• Any kidney disease or impaired renal function

• Pregnancy and breastfeeding – whilst generally considered safe for occasional use, consult a healthcare professional first

• Elderly patients – increased risk of electrolyte disturbances and reduced kidney function

• People taking certain medications – including antibiotics (tetracyclines, quinolones), bisphosphonates, levothyroxine, iron supplements, mycophenolate, and some cardiac and antiviral medications

Always inform your pharmacist or GP about all medications and supplements you take before using milk of magnesia to avoid potential interactions.

When to Seek Medical Advice

Whilst milk of magnesia is suitable for self-care management of minor digestive complaints, certain situations require professional medical evaluation. Recognising when to seek help ensures appropriate treatment and prevents delays in diagnosing potentially serious conditions.

Consult your GP if:

• Constipation persists for more than 3 days despite using milk of magnesia as directed

• You experience recurrent constipation requiring regular laxative use – this may indicate underlying conditions such as irritable bowel syndrome, hypothyroidism, or medication side effects that need investigation

• Heartburn or indigestion occurs more than twice weekly or persists despite antacid use – this may warrant investigation for gastro-oesophageal reflux disease (GORD) or other conditions

• You notice unexplained weight loss, particularly if accompanied by digestive symptoms

• Blood appears in stools or vomit, or stools become black and tarry

• You experience persistent abdominal pain, especially if severe or accompanied by fever

• There is a change in bowel habits lasting more than a few weeks, particularly in people over 50

Seek urgent medical attention (call 999 or go to A&E) if you experience:

• Severe abdominal pain that is constant or worsening

• Signs of severe allergic reaction (anaphylaxis): difficulty breathing, swelling of face or throat, rapid pulse

• Symptoms of magnesium toxicity: severe muscle weakness, confusion, very slow or irregular heartbeat, difficulty breathing

• Severe or bloody diarrhoea with signs of dehydration (dizziness, reduced urination, extreme thirst)

NICE guidance recommends that people with new-onset digestive symptoms, particularly those over 50 or with alarm features (bleeding, weight loss, anaemia), should be assessed promptly to exclude serious pathology including malignancy. Do not continue self-treating with over-the-counter remedies if symptoms persist or worsen, as this may delay important investigations. Your GP can arrange appropriate tests, provide diagnosis, and recommend evidence-based treatment tailored to your specific condition.

Frequently Asked Questions