Which magnesium makes you poop? Certain magnesium supplements exert a laxative effect by drawing water into the intestines, softening stools and promoting bowel movements. Magnesium oxide and magnesium hydroxide (milk of magnesia) are the most effective forms for constipation relief due to their poor absorption, whilst magnesium citrate has a moderate effect. Understanding the differences between magnesium formulations helps you choose the right option for occasional constipation or nutritional supplementation, whilst recognising when medical advice is needed for persistent bowel problems.

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. Poorly absorbed magnesium salts tend to remain in the intestine 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.

It is important to understand that whilst magnesium can effectively relieve occasional constipation, the laxative effect is not experienced equally with all magnesium formulations. Some forms are specifically designed to minimise gastrointestinal side effects and may be better suited for addressing magnesium deficiency. The relationship between absorption and laxative effect varies by the specific magnesium salt, dosage, and individual response.

It's worth noting that in UK clinical practice, magnesium salts are not typically recommended as first-line treatments for chronic constipation. The NHS and NICE guidance generally recommend macrogols (polyethylene glycol) or lactulose as initial treatments for ongoing constipation, with magnesium preparations more commonly used for occasional or short-term relief.

Which Magnesium Supplements Have a Laxative Effect

Magnesium oxide is one of the most commonly available and least expensive forms of magnesium supplementation. It has relatively poor bioavailability, meaning a significant proportion remains unabsorbed in the intestinal tract. This makes magnesium oxide particularly effective as a laxative, and it is frequently used for short-term relief of constipation. However, it is not the optimal choice for correcting magnesium deficiency due to its limited absorption.

Magnesium hydroxide, sold as milk of magnesia, is another poorly absorbed form with pronounced laxative properties. It has been used for generations as both an antacid and a laxative. When taken in therapeutic doses, it typically produces a bowel movement within 2-6 hours according to UK product information. This formulation is particularly useful for acute constipation but should not be used long-term without medical supervision.

Magnesium citrate has moderate bioavailability and a less pronounced laxative effect than oxide or hydroxide forms. In the UK, magnesium citrate is most commonly used as part of licensed bowel preparation products (often combined with sodium picosulfate) before colonoscopy or surgery, which should only be used under medical direction. At standard supplemental doses, magnesium citrate may cause mild loosening of stools in some individuals.

Magnesium sulphate (Epsom salts) is primarily used externally in baths. While medicinal-grade magnesium sulphate can be used as a potent oral laxative, this should only be done under appropriate medical supervision due to risks of electrolyte imbalance. Non-medicinal Epsom salts sold for bathing should never be taken orally.

Other forms such as magnesium glycinate, magnesium threonate, and magnesium taurate are often marketed as being better absorbed and less likely to cause laxative effects at recommended doses. While some evidence suggests these forms may be gentler on the digestive system, individual responses vary, and higher doses of any magnesium form can potentially cause loose stools.

Recommended Doses and How to Take Magnesium Safely

The recommended daily intake of magnesium for adults in the UK is 300 mg for men and 270 mg for women, according to NHS guidance. However, when using magnesium specifically for its laxative effect, doses differ from those used for nutritional supplementation.

For occasional constipation relief, UK-licensed magnesium hydroxide products (such as milk of magnesia) typically recommend 2.4-4.8 g of the suspension at bedtime for adults. Always follow the specific product instructions or seek advice from a pharmacist, as formulations vary in strength.

When taking magnesium supplements, several safety considerations are important:

• Start with a lower dose and gradually increase if needed, as individual tolerance varies considerably. This approach helps minimise the risk of diarrhoea and abdominal cramping.

• Take magnesium with food to reduce gastrointestinal discomfort, though this may slightly delay the laxative effect.

• Maintain adequate hydration when using magnesium for constipation, as the osmotic mechanism requires sufficient fluid intake to work effectively.

• Avoid exceeding 400 mg of supplemental elemental magnesium daily from non-food sources on a long-term basis without medical supervision, as per NHS and Expert Group on Vitamins and Minerals guidance. The European Food Safety Authority sets a lower upper limit of 250 mg/day from supplements. Higher doses should only be used short-term for constipation relief according to product instructions.

Important contraindications and cautions include individuals with kidney disease, who should not take magnesium supplements without medical advice, as impaired renal function can lead to dangerous magnesium accumulation (hypermagnesaemia). People with myasthenia gravis, significant heart block, or who are elderly or frail should also exercise caution.

Magnesium can interact with several medications, including bisphosphonates, antibiotics (particularly tetracyclines and quinolones), levothyroxine, iron supplements, dolutegravir and other integrase inhibitors, and some diuretics. Generally, separate magnesium intake from these medications by 2-4 hours. If you take regular medications, consult your GP or pharmacist before starting magnesium supplementation.

Symptoms of excessive magnesium intake include diarrhoea, nausea, abdominal cramping, and in severe cases, irregular heartbeat or muscle weakness. Report any suspected adverse reactions to magnesium-containing medicines or supplements via 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 like magnesium, certain circumstances warrant medical evaluation. You should contact your GP if you experience constipation lasting more than three weeks despite self-care measures, as this may indicate an underlying condition requiring investigation.

Red flag symptoms that require prompt medical attention include:

• Unexplained weight loss accompanying constipation

• Blood in your stools or rectal bleeding, particularly if you are aged 50 or over

• Severe abdominal pain or persistent bloating

• Alternating constipation and diarrhoea

• A change in bowel habit that is persistent and unexplained, particularly if you are aged 60 or over

• A family history of bowel cancer or inflammatory bowel disease

According to NICE guidelines (NG12), these symptoms may necessitate urgent investigation, potentially including blood tests, faecal immunochemical testing (FIT), or referral for colonoscopy to exclude serious pathology such as colorectal cancer or inflammatory bowel disease.

Seek same-day medical attention if you experience symptoms suggesting bowel obstruction (severe pain with vomiting and inability to pass stool or gas), black/tarry stools, or constipation with significant systemic illness.

Additionally, if you find yourself regularly needing laxatives (including magnesium) for more than a week or two, this suggests your constipation requires proper medical assessment. Your GP can investigate potential causes such as hypothyroidism, irritable bowel syndrome, medication side effects, or dietary insufficiencies, and recommend appropriate management strategies.

Pregnant women experiencing constipation should also consult their midwife or GP before using magnesium supplements, as dosing recommendations differ during pregnancy. Similarly, parents should seek medical advice before giving magnesium supplements to children, as paediatric dosing requires careful calculation based on age and weight.

Frequently Asked Questions