Milk of magnesia magnesium 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 for over a century as a trusted remedy for constipation and indigestion. The magnesium works through distinct mechanisms: as an osmotic laxative, it draws water into the bowel to soften stools and stimulate movement, whilst as an antacid, it neutralises excess stomach acid to relieve heartburn. Understanding how to use milk of magnesia safely, recognising when medical advice is needed, and being aware of potential interactions ensures effective symptom management whilst minimising risks.

What Is Milk of Magnesia and How Does Magnesium Work?

Milk of magnesia is an over-the-counter medicine containing magnesium hydroxide as its active ingredient. It appears as a white, chalky suspension and has been used for over a century to treat constipation and indigestion. In the UK, it is available without prescription from pharmacies and supermarkets, with legal classification varying by product and pack size (either General Sales List or Pharmacy medicine).

The magnesium in milk of magnesia works through two distinct mechanisms depending on the condition being treated. As a laxative, magnesium hydroxide draws water into the bowel through osmosis, softening the stool and stimulating bowel movements. This osmotic effect typically produces results within 2–6 hours after administration. As an antacid, it neutralises excess stomach acid by reacting with hydrochloric acid to form magnesium chloride and water, providing relief from heartburn and indigestion.

Magnesium hydroxide is minimally absorbed from the gastrointestinal tract, which means most of its action remains local within the digestive system. This limited absorption contributes to its safety profile for short-term use in individuals with normal kidney function.

The concentration of magnesium hydroxide in milk of magnesia products varies, but typical formulations contain around 415 mg of magnesium hydroxide per 5 ml of suspension. Different brands may have slightly different strengths, so it is important to read the product label carefully and follow the recommended dosage instructions. The medicine is primarily available in liquid form in the UK.

Uses and Benefits of Milk of Magnesia

Milk of magnesia has two primary licensed indications in the UK: relief of occasional constipation and treatment of indigestion and heartburn. For constipation, it is particularly useful for providing short-term relief when dietary measures and increased fluid intake have not been sufficient. The osmotic laxative effect can help restore normal bowel function when used appropriately for short periods.

As an antacid, milk of magnesia provides symptomatic relief from acid-related dyspepsia, heartburn, and gastric discomfort. It is especially helpful for occasional symptoms that occur after eating certain foods or during periods of stress. Unlike some other antacids, magnesium-based products tend to have a laxative rather than constipating effect, which can be advantageous for some patients.

Key benefits include:

• Dual action – can address both constipation and indigestion, making it versatile for digestive complaints

• Rapid onset – antacid effects occur within minutes; laxative effects typically within hours

• Non-systemic action – limited absorption means fewer systemic effects in healthy individuals

• Widely available – accessible over-the-counter option

• Established use – recognised treatment option for appropriate short-term use

According to NICE guidance on constipation management, osmotic laxatives can be considered as part of a stepped approach to treatment, though macrogols are often recommended as first-line osmotic agents. NICE emphasises that lifestyle modifications, including adequate fluid intake, dietary fibre, and physical activity, should form the foundation of constipation management. For indigestion, NICE recommends lifestyle advice as first-line management, with antacids reserved for symptomatic relief when needed.

Milk of magnesia should not be used as a long-term solution without medical supervision. If symptoms persist, this may indicate an underlying condition requiring investigation. Seek medical advice promptly if you experience difficulty swallowing, persistent vomiting, unintentional weight loss, blood in stools, or severe abdominal pain.

How to Take Milk of Magnesia Safely

Dosage varies depending on the indication and patient age. For adults and children over 12 years, the typical dose for constipation is 30–45 ml of liquid suspension taken at bedtime with water. For antacid use, the usual dose is 5–10 ml taken up to four times daily, preferably after meals and at bedtime. Always shake the bottle thoroughly before use, as the magnesium hydroxide settles at the bottom.

For children aged 3–12 years, reduced doses apply, and it is essential to consult the product information leaflet or seek advice from a pharmacist or GP before administration. Milk of magnesia is not generally recommended for children under 3 years without medical supervision. When using the medicine as a laxative, it should not be taken for more than three consecutive days without consulting a healthcare professional.

Important administration guidance:

• Take with a full glass of water to aid the osmotic effect and prevent dehydration

• If using as an antacid, take after meals and at bedtime for optimal effect

• If using as a laxative, bedtime dosing is often preferred as it allows overnight action

• Do not exceed the recommended dose, as this increases the risk of adverse effects

• Maintain adequate fluid intake throughout the day, especially when using as a laxative

Milk of magnesia can interact with other medications by affecting their absorption. It should be taken at least 2–3 hours apart from other oral medicines, including antibiotics (particularly tetracyclines and quinolones), bisphosphonates, iron supplements, levothyroxine, and certain cardiac medications. If you are taking regular prescription medicines, consult your pharmacist about appropriate timing.

When to seek medical advice: If constipation persists beyond three days of treatment, or if you experience severe abdominal pain, nausea, vomiting, or rectal bleeding, contact your GP promptly. Similarly, if indigestion symptoms continue despite treatment or are accompanied by unexplained weight loss, difficulty swallowing, persistent vomiting, or blood in vomit or stools, medical assessment is necessary to exclude serious underlying conditions.

Side Effects and Precautions

Milk of magnesia is generally well tolerated when used as directed for short periods. However, like all medicines, it can cause side effects in some individuals. The most common adverse effects relate to its action on the gastrointestinal system and include diarrhoea, abdominal cramps, nausea, and flatulence. These effects are usually mild and resolve once treatment is stopped.

Diarrhoea is the most frequently reported side effect, particularly when doses exceed recommendations or when the medicine is used for prolonged periods. Excessive diarrhoea can lead to dehydration and electrolyte imbalances, which may be particularly problematic in elderly patients or those with underlying health conditions. If severe or persistent diarrhoea occurs, discontinue use and seek medical advice.

Magnesium accumulation is a rare but serious concern, particularly in individuals with impaired kidney function. Symptoms of hypermagnesaemia (elevated blood magnesium levels) include muscle weakness, confusion, irregular heartbeat, low blood pressure, and in severe cases, respiratory depression. These effects are uncommon with short-term use in people with normal renal function but warrant caution in at-risk groups.

Precautions to observe:

• Avoid prolonged use without medical supervision

• Monitor for signs of dehydration, especially in hot weather or during illness

• Be aware that chronic use may lead to electrolyte disturbances, including low calcium and potassium levels

• Patients with inflammatory bowel disease should consult their doctor before use

• Those on sodium-restricted diets should check product labels, as some formulations contain sodium

Drug interactions are an important consideration. Magnesium can reduce the absorption of numerous medications, including levothyroxine, digoxin, bisphosphonates, and certain antibiotics and antifungal agents. The chelating properties of magnesium can bind to these drugs in the gastrointestinal tract, reducing their bioavailability and therapeutic effect. Always inform your pharmacist or GP about all medications you are taking, including over-the-counter products and supplements.

Contact your GP if you experience: persistent or worsening symptoms, signs of electrolyte imbalance (muscle weakness, irregular heartbeat, confusion), severe abdominal pain, blood in stools, or unexplained weight loss. These may indicate conditions requiring further investigation or alternative management strategies.

If you experience any side effects, talk to your doctor, pharmacist or nurse. This includes any possible side effects not listed in the product packaging. You can also report side effects directly via the Yellow Card Scheme at yellowcard.mhra.gov.uk.

Who Should Avoid Milk of Magnesia?

Certain individuals should not use milk of magnesia, or should only do so under close medical supervision. Absolute contraindications include severe renal impairment or kidney failure, as the reduced ability to excrete magnesium significantly increases the risk of dangerous magnesium accumulation in the blood. Patients with myasthenia gravis should consult their doctor before using magnesium-containing products, as there is a theoretical risk that magnesium could affect neuromuscular function.

Individuals with intestinal obstruction, acute abdominal conditions, or appendicitis should not use milk of magnesia, as laxatives may exacerbate these serious conditions or mask important symptoms requiring urgent medical attention. Similarly, those with inflammatory bowel disease (Crohn's disease or ulcerative colitis) should exercise caution and seek medical advice before use, particularly during an acute flare-up.

Pregnancy and breastfeeding: Milk of magnesia is generally considered safe for short-term, occasional use during pregnancy and breastfeeding, as systemic absorption is limited. However, pregnant women should consult their midwife or GP before use, particularly in the first trimester. For breastfeeding mothers, small amounts of magnesium may pass into breast milk, but this is not expected to harm the infant when used occasionally at recommended doses.

Elderly patients require special consideration due to age-related decline in kidney function, increased risk of dehydration, and potential for polypharmacy interactions. Lower doses may be appropriate, and medical advice should be sought if symptoms persist.

Additional groups requiring caution:

• Patients taking medications that may interact with magnesium hydroxide

• Individuals with electrolyte imbalances or on diuretic therapy

• Those with severe dehydration

• Patients with diabetes (some formulations contain sugar)

• Anyone with hypersensitivity to any ingredients in the product

If you have any chronic medical conditions or take regular prescription medicines, consult your GP or pharmacist before using milk of magnesia. They can assess whether it is appropriate for your individual circumstances and advise on safe use alongside your existing treatment regimen. Remember that persistent digestive symptoms warrant medical evaluation to identify and address any underlying causes rather than relying on ongoing symptomatic treatment.

Frequently Asked Questions