Does magnesium give you the runs? Yes, magnesium supplements commonly cause diarrhoea, making it one of the most frequently reported side effects of supplementation. This gastrointestinal effect occurs because magnesium acts as an osmotic laxative, drawing water into the bowel and softening stools. The likelihood and severity depend on the type of magnesium compound, dosage, and individual tolerance. Whilst dietary magnesium from foods rarely causes problems, concentrated supplement forms—particularly magnesium oxide and citrate—are more likely to trigger loose stools. Understanding which forms are better tolerated and how to take magnesium properly can help minimise this unwanted effect whilst still achieving adequate intake.

Does Magnesium Give You Diarrhoea?

Yes, magnesium supplements can cause diarrhoea, and this is one of the most commonly reported side effects associated with magnesium supplementation. The likelihood and severity of this gastrointestinal effect depend on several factors, including the type of magnesium compound used, the dosage taken, and individual tolerance levels.

Magnesium acts as an osmotic laxative when present in the gastrointestinal tract in sufficient quantities. This means it draws water into the bowel, softening stools and potentially accelerating transit time through the intestines. For some individuals, even modest supplementation can trigger loose stools or diarrhoea, whilst others may tolerate higher doses without significant gastrointestinal upset.

The threshold dose at which magnesium causes diarrhoea varies considerably between individuals. According to NHS guidance, doses exceeding 400mg of elemental magnesium daily are more likely to produce laxative effects, though some people experience symptoms at lower doses. The European Food Safety Authority (EFSA) has set a lower safe upper limit for supplemental magnesium at 250mg daily for adults, specifically to minimise the risk of gastrointestinal side effects.

People with kidney disease should be particularly cautious with magnesium supplements, as they may be at risk of hypermagnesaemia (high blood magnesium levels). Older adults may also be more sensitive to the laxative effects.

It is worth noting that magnesium obtained through dietary sources—such as green leafy vegetables, nuts, seeds, and whole grains—rarely causes diarrhoea, as the magnesium is present in lower concentrations and is absorbed more gradually. The issue predominantly arises with concentrated supplement forms, particularly when taken on an empty stomach or in single large doses rather than divided throughout the day.

If you experience persistent diarrhoea (lasting more than 7 days), severe abdominal pain, blood in your stools, fever, or signs of dehydration, you should contact your GP or NHS 111 promptly.

Which Types of Magnesium Are Most Likely to Cause Diarrhoea

Not all magnesium supplements are created equal when it comes to gastrointestinal tolerability. The chemical form of magnesium significantly influences both its absorption characteristics and its propensity to cause loose stools.

Magnesium oxide is amongst the most poorly absorbed forms and consequently has a stronger laxative effect. It has relatively low bioavailability compared to other forms, meaning a significant proportion of ingested magnesium oxide remains in the intestinal lumen, drawing water into the bowel and potentially triggering diarrhoea. Despite this, it remains widely available due to its low cost and high elemental magnesium content per tablet.

Magnesium citrate and magnesium sulphate (Epsom salts) are also notable for their laxative properties. In the UK, magnesium citrate is used in some bowel preparation products (such as Citramag) before certain medical procedures. Whilst it has better bioavailability than magnesium oxide, it still commonly causes loose stools, particularly at higher doses.

Magnesium chloride and magnesium hydroxide (milk of magnesia) similarly have pronounced laxative effects. According to the British National Formulary (BNF), magnesium hydroxide is specifically licensed in the UK as a laxative and antacid, making it unsuitable for those seeking supplementation without gastrointestinal effects.

Some evidence suggests that chelated forms such as magnesium glycinate, magnesium taurate, and magnesium threonate may be better tolerated by some individuals. These forms may have different absorption profiles, with potentially more magnesium being absorbed in the small intestine rather than remaining in the bowel. Magnesium malate may also be associated with relatively fewer gastrointestinal side effects compared to oxide or citrate forms for some people, though high-quality comparative studies are limited.

How to Take Magnesium Without Getting the Runs

Several practical strategies can help minimise the laxative effects of magnesium supplementation whilst still achieving adequate intake.

Consider the form: Some people find that forms such as magnesium glycinate or magnesium malate may be better tolerated than magnesium oxide or citrate, though individual responses vary.

Start with a low dose and increase gradually: Beginning with a lower dose (such as 100–150mg of elemental magnesium daily) and slowly increasing over several weeks may help your digestive system adapt. This cautious approach can help identify your personal tolerance threshold. Sudden introduction of high doses is more likely to trigger diarrhoea.

Divide your daily dose: Rather than taking your entire magnesium supplement in one go, split it into two or three smaller doses throughout the day. This reduces the concentration of magnesium in the intestines at any given time and may improve overall absorption.

Take magnesium with food: Consuming magnesium supplements alongside meals slows gastric emptying and can reduce gastrointestinal irritation. The presence of food also promotes more gradual absorption.

Stay within recommended limits: The NHS advises that adults should not exceed 400mg of supplemental magnesium daily, while EFSA recommends a lower limit of 250mg daily. Consider staying well below these thresholds—particularly when first starting supplementation—to reduce the risk of adverse effects.

Be aware of interactions: Magnesium supplements can reduce the absorption of certain medications, including tetracycline and quinolone antibiotics, levothyroxine, and bisphosphonates. Separate these medications from magnesium supplements by at least 2-4 hours.

Special precautions: If you have kidney disease, are pregnant or breastfeeding, or take other medications, consult your GP or pharmacist before taking magnesium supplements.

If diarrhoea persists despite these measures, discontinue supplementation and consult your GP. Persistent diarrhoea can lead to dehydration and electrolyte imbalances, and may indicate an underlying condition requiring investigation. Seek medical advice promptly if you experience severe or prolonged diarrhoea, blood in your stools, fever, or signs of dehydration.

If you suspect side effects from a magnesium-containing medicine, you can report them via the MHRA Yellow Card scheme.

Why Magnesium Can Cause Loose Stools

Understanding the mechanism behind magnesium-induced diarrhoea helps explain why this side effect is so common and dose-dependent.

Magnesium functions as an osmotic laxative when present in the intestinal lumen in concentrations that exceed the colon's absorptive capacity. Osmotic laxatives work by retaining water within the bowel through osmotic pressure—essentially, the magnesium molecules attract and hold water, preventing its reabsorption through the intestinal wall. This increased fluid content softens the stool and stimulates peristalsis (the wave-like muscle contractions that move contents through the digestive tract), resulting in more frequent and looser bowel movements.

The absorption of different magnesium compounds is central to this effect. Forms with poorer absorption—such as magnesium oxide—leave more unabsorbed magnesium in the colon. This unabsorbed magnesium continues to exert osmotic effects, drawing water into the bowel and potentially triggering diarrhoea. Conversely, forms with different absorption profiles may leave less residual magnesium in the bowel.

Individual variation in magnesium absorption also plays a role. Factors such as gastric acid levels, intestinal transit time, the presence of other nutrients, and the composition of the gut microbiome all influence how much magnesium is absorbed versus how much remains in the bowel. Some individuals naturally absorb magnesium less efficiently and are therefore more susceptible to its laxative effects.

It is worth noting that whilst the laxative effect is generally considered a side effect in the context of supplementation, it is deliberately exploited in clinical settings—magnesium-containing preparations are used therapeutically to treat constipation and prepare the bowel for colonoscopy or surgery.

People with kidney problems should be particularly cautious with magnesium supplements, as reduced renal function can lead to accumulation of magnesium in the blood (hypermagnesaemia), which can be dangerous. Always seek medical advice before taking magnesium supplements if you have kidney disease.

Frequently Asked Questions