Which magnesium makes you poop? Certain magnesium supplements act as osmotic laxatives by drawing water into the intestines, softening stools and promoting bowel movements. Magnesium hydroxide (milk of magnesia) is a UK-licensed laxative that typically works within 2–4 hours. Magnesium oxide and citrate may also have laxative effects due to poor absorption, whilst better-absorbed forms like magnesium glycinate are less likely to cause loose stools. Understanding which formulation suits your needs—and when to seek medical advice—is essential for safe, effective use.

How Magnesium Affects Bowel Movements

Magnesium plays a crucial role in numerous bodily functions, including the regulation of bowel movements. When taken orally, certain forms of magnesium exert an osmotic laxative effect within the gastrointestinal tract. This mechanism works by drawing water into the intestinal lumen through osmosis, which softens the stool and increases its volume. The increased water content stimulates peristalsis—the wave-like muscular contractions that propel faecal matter through the colon—thereby promoting bowel movements.

The laxative effect of magnesium is dose-dependent and varies significantly depending on the chemical form of the supplement. Magnesium salts that are less well absorbed tend to remain in the intestinal tract longer, creating a greater osmotic gradient and consequently a more pronounced laxative action. This is why magnesium has been used medicinally for constipation relief for many decades, with magnesium hydroxide (commonly known as milk of magnesia) being a traditional over-the-counter remedy in the UK.

It is important to understand that whilst magnesium can effectively relieve occasional constipation, the laxative effect is not experienced uniformly across all magnesium supplements. Some forms are better absorbed and may have less pronounced effects on bowel movements. The relationship between absorption and laxative effect is complex and depends on the specific formulation, dose, and individual factors.

Individual response to magnesium varies considerably based on factors including gut health, hydration levels, and concurrent medications. The NHS recommends discussing persistent constipation with a healthcare professional rather than relying on self-medication long-term.

Which Magnesium Supplements Have a Laxative Effect

Not all magnesium supplements are created equal when it comes to their effect on bowel movements. Magnesium hydroxide (milk of magnesia) is a licensed medicinal product in the UK specifically for constipation relief. It works as an osmotic laxative and is available over the counter as an oral suspension. According to the British National Formulary (BNF), it typically produces a bowel movement within 2-4 hours of administration.

Magnesium oxide is commonly available as a dietary supplement but is not specifically licensed as a laxative in the UK. However, it is relatively poorly absorbed compared to some other forms, which means more remains in the intestines where it can exert an osmotic effect. This may result in looser stools when taken at higher doses.

Magnesium citrate is available in the UK as a dietary supplement. In medical settings, prescription-only preparations containing magnesium citrate (such as CITRAMAG or combinations with sodium picosulfate) are used for bowel preparation before colonoscopy procedures. These prescription products should only be used under medical supervision.

It's important to note that Epsom salts (magnesium sulphate) sold for bathing should never be taken orally. Any oral magnesium sulphate product would require appropriate medicinal licensing and should only be used as directed by a healthcare professional.

Other magnesium forms such as magnesium glycinate and magnesium malate are generally better absorbed and may be less likely to cause loose stools. These forms are typically marketed as dietary supplements rather than laxatives.

For constipation, NICE guidelines recommend macrogols (polyethylene glycol laxatives) as first-line treatment. Magnesium-based laxatives are an alternative option, but should be used according to product licensing and instructions. If you're unsure which product is appropriate, consult a pharmacist for advice on UK-licensed options.

Recommended Doses and How to Take Magnesium Safely

The appropriate dose of magnesium depends on whether you are using it for nutritional supplementation or as a laxative for constipation relief. The UK Reference Nutrient Intake (RNI) for magnesium is 300 mg daily for men and 270 mg daily for women. For nutritional purposes, the NHS advises that magnesium supplements should not exceed the RNI without medical advice.

For occasional constipation relief, always follow the dosing instructions on the specific UK-licensed product you are using. For example, magnesium hydroxide mixture (milk of magnesia) typically recommends 5-10 ml for adults and children over 12 years, taken preferably at bedtime. The exact dosing should be checked on the product label or patient information leaflet, as formulations may vary.

Important safety considerations include:

• Hydration: Drink plenty of water when taking magnesium-based laxatives, as the osmotic effect requires adequate fluid intake to work effectively and safely.

• Kidney function: Individuals with impaired renal function should consult their GP before taking magnesium supplements or laxatives, as the kidneys are responsible for excreting excess magnesium. Accumulation can lead to hypermagnesaemia, a potentially serious condition.

• Drug interactions: Magnesium can interfere with the absorption of certain medications. Take magnesium at least 4 hours apart from levothyroxine, and at least 2-3 hours apart from tetracycline antibiotics, fluoroquinolone antibiotics, bisphosphonates, and iron supplements. Check with your pharmacist about specific medication interactions.

• Special populations: Pregnant or breastfeeding women, children, and older adults should seek medical advice before using magnesium laxatives. Those with inflammatory bowel disease or other gastrointestinal disorders should also consult a healthcare professional first.

Chronic use of magnesium as a laxative is not recommended without medical supervision, as it may mask underlying conditions requiring investigation. If you find yourself needing laxatives regularly for more than one week, consult your GP to identify and address the root cause of constipation. NICE recommends macrogols (polyethylene glycol) as first-line treatment for constipation in adults.

If you experience any adverse effects from taking magnesium products, report them through the MHRA Yellow Card Scheme.

When to See Your GP About Constipation

Whilst occasional constipation is common and often responds well to dietary changes, increased fluid intake, and over-the-counter remedies, certain circumstances warrant medical evaluation. You should contact your GP if:

• Constipation persists despite trying pharmacy treatments

• You experience unexplained weight loss alongside bowel changes

• There is blood in your stool or rectal bleeding

• You have severe abdominal pain or bloating

• Constipation alternates with diarrhoea

• You notice a persistent change in bowel habit, particularly if you are over 60 years of age

• You experience symptoms such as fatigue, weakness, or confusion (which may indicate electrolyte imbalances or other systemic issues)

• You have severe abdominal distension with vomiting and inability to pass wind (which may indicate bowel obstruction requiring urgent attention)

These symptoms may indicate conditions requiring investigation, such as colorectal cancer, inflammatory bowel disease, hypothyroidism, or medication side effects. NICE guidelines recommend urgent referral for suspected colorectal cancer in patients aged 40 or over with unexplained weight loss and abdominal pain, those aged 50 or over with unexplained rectal bleeding, and those aged 60 or over with iron-deficiency anaemia or changes in bowel habit. Your GP may use faecal immunochemical testing (FIT) as part of the assessment process.

Your GP will take a thorough history, including your diet, fluid intake, physical activity levels, and current medications. Physical examination may include abdominal palpation and, if appropriate, digital rectal examination. Depending on clinical findings, investigations might include blood tests (full blood count, thyroid function, calcium levels), and in some cases, referral for colonoscopy or other imaging.

For individuals with chronic constipation, management extends beyond laxatives to include dietary fibre optimisation (aiming for 30g daily), adequate hydration (approximately 1.5–2 litres of fluid daily), regular physical activity, and establishing consistent bowel routines. Your GP may prescribe specific laxatives, with macrogols (polyethylene glycol) being the first-line treatment recommended by NICE, or refer you to a gastroenterologist if first-line measures prove insufficient. Remember that whilst magnesium can be a useful tool for occasional constipation, it should complement—not replace—a holistic approach to bowel health.

Frequently Asked Questions