Which magnesium makes you poop? Certain magnesium supplements exert an osmotic laxative effect by drawing water into the intestines, softening stools and promoting bowel movements. Magnesium hydroxide (milk of magnesia) is the most commonly used form in the UK for constipation relief, whilst magnesium sulphate and magnesium citrate also have laxative properties. However, not all magnesium supplements cause this effect—forms such as magnesium glycinate are less likely to produce loose stools. Understanding which formulations work as laxatives helps ensure safe, effective use, whether you're seeking constipation relief or nutritional supplementation without unwanted gastrointestinal effects.

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 varies depending on the chemical form of the supplement. Less well-absorbed magnesium salts remain in the intestinal tract longer, creating a greater osmotic effect and consequently a more pronounced laxative action. This is why certain magnesium preparations have been used medicinally for constipation relief, 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 help relieve occasional constipation, it is not considered a first-line treatment in UK practice. The NHS and NICE Clinical Knowledge Summaries recommend lifestyle measures, adequate hydration, and increased dietary fibre as initial approaches, followed by macrogol laxatives if needed.

For individuals seeking constipation relief, understanding which magnesium formulations produce this effect is essential for safe and effective use. Conversely, those taking magnesium for nutritional purposes may wish to choose forms less likely to cause loose stools or abdominal discomfort.

Which Magnesium Supplements Have a Laxative Effect

Not all magnesium supplements are equally effective at promoting bowel movements. The laxative potency depends primarily on the compound's solubility and absorption characteristics. In the UK, magnesium hydroxide (milk of magnesia) is the most commonly available magnesium-based laxative. It works as both a gentle laxative and antacid, typically producing bowel movements within 6 to 12 hours of ingestion.

Magnesium sulphate (Epsom salts) is another saline laxative available in the UK, though it is less commonly recommended for regular oral use due to its bitter taste and potential for electrolyte disturbances. It should not be used repeatedly without medical supervision.

Magnesium citrate is sometimes used in clinical settings for bowel preparation before colonoscopy or surgery, but only as part of licensed, supervised regimens (such as Citramag or in combination products like Picolax). These preparations should only be used under healthcare professional guidance and are not intended for self-management of constipation.

Other magnesium forms such as magnesium glycinate, magnesium malate, and magnesium taurate are generally less likely to cause a pronounced laxative effect. These forms are more commonly taken as nutritional supplements rather than for constipation relief. However, even these forms may cause loose stools in some individuals, particularly at higher doses.

It's worth noting that in the UK, osmotic laxatives such as macrogols (polyethylene glycol) are typically recommended as first-line treatments for constipation rather than magnesium salts, according to NICE guidance and the British National Formulary (BNF).

Recommended Doses and How to Take Magnesium Safely

The appropriate dose of magnesium depends on whether you are using it to relieve constipation or to supplement dietary intake. For constipation relief, magnesium hydroxide (milk of magnesia) is typically taken according to the product label or patient information leaflet. Always follow the specific dosing instructions on the product you are using, as formulations vary.

When using magnesium as a laxative, take it with a full glass of water to enhance the effect and prevent dehydration. It is advisable to start with the lowest recommended dose, as individual sensitivity varies considerably.

For nutritional supplementation, the Reference Nutrient Intake (RNI) for magnesium in the UK is 300 mg for men and 270 mg for women. When taking magnesium for general health rather than constipation, it is usually best taken with food to minimise gastrointestinal upset.

Important safety considerations include:

• Hydration: Drink plenty of water when taking magnesium supplements, particularly laxative forms, to prevent dehydration.

• Kidney function: Individuals with kidney disease should avoid magnesium supplements unless specifically advised by their doctor, as impaired renal function can lead to dangerous magnesium accumulation (hypermagnesaemia). Older adults should also use caution.

• Drug interactions: Magnesium can interfere with the absorption of certain medications, including bisphosphonates, tetracycline and quinolone antibiotics, levothyroxine, and oral iron. Take magnesium at least 4 hours apart from levothyroxine and bisphosphonates, and at least 2-3 hours apart from other affected medications.

• Duration of use: Magnesium laxatives should not be used for more than one week without medical advice, as chronic use may lead to dependence or electrolyte imbalances.

Adverse effects of excessive magnesium intake include diarrhoea, nausea, abdominal cramping, and in severe cases, cardiac arrhythmias or respiratory depression. If you experience persistent diarrhoea or other concerning symptoms, discontinue use and consult your GP. Report any suspected side effects to the MHRA Yellow Card Scheme.

When to See Your GP About Constipation

Whilst occasional constipation is common and often responds well to dietary changes or over-the-counter remedies, certain symptoms warrant medical evaluation. You should contact your GP promptly if you experience constipation accompanied by any of the following red flag symptoms:

• Rectal bleeding (particularly if you are aged 50 or over)

• Unexplained weight loss

• Persistent abdominal pain or bloating that does not resolve

• Persistent change in bowel habit, particularly if you are aged 60 or over

• Symptoms of bowel obstruction, such as severe cramping, vomiting, or inability to pass wind

• Iron deficiency anaemia detected on blood tests

• Abdominal or rectal mass found on examination

These symptoms may indicate underlying conditions requiring investigation, such as colorectal cancer, inflammatory bowel disease, or intestinal obstruction. According to NICE guidance (NG12), patients presenting with these features should be assessed urgently, and those meeting specific criteria may require referral via the suspected cancer pathway.

You should also seek medical advice if constipation persists despite lifestyle modifications and over-the-counter treatments, or if you find yourself increasingly reliant on laxatives. Chronic constipation may result from underlying medical conditions including hypothyroidism, diabetes, neurological disorders, or medication side effects. Your GP can perform appropriate investigations, which may include blood tests to check thyroid function, calcium levels, and full blood count.

Additionally, if you have pre-existing medical conditions such as kidney disease, heart failure, or inflammatory bowel disease, consult your GP before starting magnesium supplements. Pregnant or breastfeeding women should seek advice from a healthcare professional about the most appropriate laxative options, as some (such as macrogols) may be preferred.

For most people, constipation can be effectively managed through increased dietary fibre, adequate hydration, regular physical activity, and appropriate use of laxatives when needed. However, persistent or concerning symptoms should never be ignored, as timely medical assessment ensures appropriate diagnosis and management of any underlying pathology.

Frequently Asked Questions