Constipation affects millions of people in the UK, and magnesium-based laxatives offer a well-established treatment option. The best magnesium for constipation is magnesium hydroxide (milk of magnesia), the only magnesium preparation licensed in the UK specifically for constipation relief. This osmotic laxative works by drawing water into the bowel, softening stools and stimulating bowel movements, typically within 2–6 hours. Whilst magnesium can provide effective short-term relief, it's important to understand which forms are appropriate, how to use them safely, and when to seek medical advice for persistent symptoms.

How Magnesium Relieves Constipation

Magnesium acts as an osmotic laxative, drawing water into the intestinal lumen through osmosis. This mechanism increases the water content of stool, softening it and stimulating peristalsis—the wave-like muscular contractions that propel faecal matter through the colon. The increased luminal volume also triggers stretch receptors in the bowel wall, promoting the urge to defecate.

When magnesium salts dissolve in the gastrointestinal tract, they create an osmotic gradient that affects water absorption in the colon. This retained fluid makes stools easier to pass and reduces straining during bowel movements. The effect typically occurs within 30 minutes to 6 hours after taking a licensed magnesium laxative, depending on the specific preparation and individual factors such as gut transit time.

Magnesium also plays a physiological role in muscle function, including the smooth muscle of the intestinal wall. While some evidence suggests that adequate magnesium levels support normal bowel motility, the relationship between magnesium deficiency and constipation is not firmly established.

It is important to note that whilst magnesium laxatives can provide symptomatic relief for occasional constipation, they do not address underlying causes such as dietary fibre deficiency, inadequate fluid intake, or certain medical conditions. The NHS recommends lifestyle measures (increased fibre, fluids, and physical activity) and first-line laxatives such as macrogol or lactulose before considering other options.

The laxative effect of magnesium is dose-dependent, with higher doses producing more pronounced effects. This makes magnesium a useful option for short-term management of constipation, though long-term use should be discussed with a healthcare professional to ensure appropriate investigation of chronic symptoms and to avoid potential electrolyte imbalances.

Types of Magnesium for Constipation Relief

Not all magnesium compounds are equally effective for constipation relief, and in the UK, only certain magnesium preparations are licensed as medicines for treating constipation.

Magnesium hydroxide, commonly known as milk of magnesia, is a traditional remedy with a UK marketing authorisation for constipation relief. It works relatively quickly, typically producing bowel movements within 2-6 hours according to product information. This form is available as a liquid suspension from pharmacies.

Magnesium citrate is sometimes found in bowel preparation products used before certain medical procedures. It has osmotic effects but is not widely available as a licensed constipation treatment in the UK. Some magnesium citrate products are sold as food supplements, but these are not licensed medicines and should not be used specifically to treat constipation.

Magnesium oxide is available in various forms, including as an ingredient in some antacid preparations that may have a mild laxative effect. However, most magnesium oxide products in the UK are sold as food supplements rather than licensed laxatives.

It is important to understand that magnesium sulphate (Epsom salts) products sold for bathing should never be taken orally. While pharmaceutical-grade magnesium sulphate has medical uses in specific clinical settings, bath salts are not formulated for ingestion and are not recommended for constipation in routine UK practice.

Other forms such as magnesium glycinate and magnesium malate are better absorbed and less likely to cause diarrhoea, making them more suitable for nutritional supplementation rather than treating constipation.

For constipation relief, it is advisable to use only products that have a UK marketing authorisation for this specific purpose. A pharmacist can help you identify appropriate licensed medicines for constipation.

Magnesium Products for Constipation in the UK

In the UK, magnesium hydroxide oral suspension (milk of magnesia) is the main magnesium-based product licensed as a medicine for constipation relief. It is available from pharmacies as a General Sale List (GSL) medicine, meaning it can be purchased without a prescription.

When selecting a magnesium product for constipation, look specifically for medicines with a marketing authorisation from the Medicines and Healthcare products Regulatory Agency (MHRA). These products will have a Product Licence (PL) number on the packaging and come with a Patient Information Leaflet (PIL) that provides important safety information and dosing instructions.

Magnesium hydroxide is available as a liquid suspension and is licensed for the short-term relief of occasional constipation. The liquid format allows for flexible dosing according to the product's instructions. Some preparations may also contain other ingredients for additional effects.

It is important to distinguish between licensed medicines and food supplements. Many magnesium products (including citrate powders, oxide tablets, and various chelated forms) are sold as food supplements. These products are regulated as foods rather than medicines and are intended for nutritional supplementation, not for treating medical conditions like constipation.

When purchasing a magnesium product for constipation relief, speak with a pharmacist who can advise on the most appropriate option based on your symptoms and medical history. They can also discuss other laxative options that may be more suitable, such as macrogol or lactulose, which are often recommended as first-line treatments for constipation in UK practice.

Always read the label and patient information leaflet carefully, and follow the manufacturer's dosing instructions unless advised otherwise by a healthcare professional.

Recommended Dosage and How to Take Magnesium

For constipation relief, dosage should be based on the specific licensed product being used. For magnesium hydroxide (milk of magnesia), the typical adult dose is 20-45 ml of liquid suspension, usually taken at bedtime, though this may vary between products. Always follow the specific dosing instructions on the product label or Patient Information Leaflet.

It is advisable to start with the lower end of the dosage range and increase gradually if needed, as individual tolerance varies considerably. Taking magnesium with food may reduce gastrointestinal discomfort, though this can also slow the laxative effect.

Timing is important for optimal results. Many people find that taking magnesium hydroxide in the evening produces a bowel movement the following morning, fitting conveniently into daily routines. The effect typically begins within 2-6 hours after taking the medicine.

Ensure adequate fluid intake when using magnesium for constipation—aim for at least 6–8 glasses of water daily. This supports the osmotic mechanism and prevents dehydration, which can paradoxically worsen constipation.

Magnesium laxatives should not be used continuously for more than one week without medical advice. Chronic constipation requires proper investigation to identify underlying causes such as irritable bowel syndrome, hypothyroidism, or medication side effects. If you require regular laxative use, consult your GP to discuss appropriate management strategies.

Certain medications may interact with magnesium, including antibiotics (particularly tetracyclines and quinolones), bisphosphonates, levothyroxine, and iron supplements. Take magnesium at least 2–4 hours apart from these medicines to avoid reduced absorption. If you are taking any regular medications, check with your pharmacist or doctor before using magnesium products.

Side Effects and Safety Considerations

Whilst magnesium is generally well-tolerated when used appropriately, excessive intake can cause adverse effects. The most common side effect is diarrhoea, which occurs when the osmotic load exceeds the colon's capacity to regulate water absorption. This is dose-dependent and usually resolves when the dose is reduced.

Other gastrointestinal effects may include:

• Abdominal cramping or discomfort

• Nausea

• Bloating

• Urgency to defecate

These symptoms are typically mild and transient, but if they persist or become severe, discontinue use and seek advice from a healthcare professional.

Hypermagnesaemia (elevated blood magnesium levels) is rare in people with normal kidney function, as excess magnesium is efficiently excreted in urine. However, individuals with chronic kidney disease should avoid magnesium supplements unless specifically prescribed and monitored by a nephrologist, as impaired renal clearance can lead to dangerous accumulation.

Symptoms of magnesium toxicity include muscle weakness, confusion, irregular heartbeat, and difficulty breathing. This is a medical emergency requiring immediate attention, though it is extremely uncommon with oral supplementation in healthy individuals.

For pregnant or breastfeeding women experiencing constipation, the NHS generally recommends lifestyle changes first, followed by macrogol or lactulose as first-line laxatives. Consult your midwife or GP before using magnesium laxatives during pregnancy or breastfeeding.

Contraindications include intestinal obstruction, acute surgical abdomen, and severe inflammatory conditions of the digestive tract. Older adults and those at risk of electrolyte imbalances should use magnesium laxatives with caution.

If you experience any unexpected side effects when taking a magnesium product, report them through the MHRA Yellow Card scheme, which helps monitor the safety of medicines in the UK.

When to See Your GP About Constipation

Whilst occasional constipation is common and often responds to lifestyle measures and over-the-counter remedies, certain symptoms warrant medical evaluation. You should consult your GP if you experience:

• Constipation lasting more than three weeks despite self-care measures

• Blood in your stools (either bright red or dark, tarry stools)

• Unexplained weight loss accompanying bowel changes

• Severe abdominal pain or persistent bloating

• A change in bowel habit that is unusual for you, particularly if you are over 60

• Alternating constipation and diarrhoea

According to NICE guidelines (NG12), certain symptoms may require urgent investigation. Your GP should consider an urgent suspected cancer referral (seen within two weeks) if you are aged 60 or over with unexplained changes in bowel habit, aged 50 or over with unexplained rectal bleeding, or aged 40 or over with unexplained weight loss and abdominal pain.

Your GP may arrange a Faecal Immunochemical Test (FIT) to help assess your risk of colorectal cancer if you have symptoms that don't meet the criteria for immediate referral.

Seek urgent same-day medical attention if you experience severe abdominal pain with vomiting and inability to pass wind or stool, as these may indicate bowel obstruction.

Chronic constipation may also result from medications including opioid analgesics, certain antidepressants, iron supplements, and some antihypertensives. Your GP can review your medication regimen and suggest alternatives if appropriate.

If you find yourself relying on laxatives (including magnesium) regularly, this suggests an underlying problem that requires proper assessment. Your doctor may arrange investigations such as blood tests to check thyroid function and calcium levels, or refer you for further tests if indicated.

For children experiencing constipation, seek medical advice rather than self-treating with magnesium products. Paediatric constipation requires tailored management approaches that consider the specific physiological needs and safety profiles of children.

Frequently Asked Questions