Does magnesium cause diarrhoea? Yes, magnesium supplements commonly cause diarrhoea, making it one of the most frequently reported adverse effects of supplementation. Magnesium acts as an osmotic laxative, drawing water into the bowel and increasing motility. The likelihood of experiencing diarrhoea depends on the dose, the specific chemical form of magnesium, and individual tolerance. Whilst some formulations are better tolerated than others, understanding how to select appropriate supplements and adjust dosing can help minimise gastrointestinal side effects whilst still obtaining the benefits of magnesium for health.

Does Magnesium Cause Diarrhoea?

Yes, magnesium supplements can cause diarrhoea, and this is one of the most commonly reported adverse effects associated with magnesium supplementation. The likelihood and severity of diarrhoea depend on several factors, including the dose taken, the specific form of magnesium used, 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 increasing bowel motility. Whilst this effect can be therapeutically beneficial for individuals with constipation, it becomes problematic when it leads to loose stools or diarrhoea in those seeking magnesium for other health reasons.

Magnesium-containing medicines list diarrhoea as a common side effect in their patient information leaflets. Some people may experience loose stools even at lower supplemental doses, with the risk increasing at higher doses.

Key factors influencing whether magnesium causes diarrhoea include:

• The chemical form of magnesium (some are more poorly absorbed and therefore more likely to cause osmotic effects)

• The total daily dose and whether it is taken all at once or divided throughout the day

• Individual gastrointestinal sensitivity and baseline bowel habits

• Whether the supplement is taken with or without food

• Kidney function (those with impaired kidney function should avoid magnesium supplements unless advised by a doctor)

The UK Reference Nutrient Intake (RNI) for magnesium is 300 mg daily for men and 270 mg daily for women, with a guidance level of 400 mg daily for supplemental magnesium (excluding dietary sources).

If you experience persistent diarrhoea whilst taking magnesium supplements, it is advisable to reduce the dose, switch to a different formulation, or consult your GP to rule out other causes and discuss whether supplementation remains appropriate for your circumstances.

Which Forms of Magnesium Are Less Likely to Cause Diarrhoea?

Not all magnesium supplements are created equal when it comes to gastrointestinal tolerability. The chemical form of magnesium influences both its absorption rate and its propensity to cause diarrhoea. Forms with higher bioavailability are generally better absorbed in the small intestine, leaving less unabsorbed magnesium to exert osmotic effects in the colon.

Magnesium glycinate is often reported to be well-tolerated. In this chelated form, magnesium is bound to the amino acid glycine, which may enhance absorption and reduce the osmotic load in the bowel. Many patients who cannot tolerate other forms report better tolerance with magnesium glycinate.

Magnesium citrate offers moderate absorption and is generally better tolerated than magnesium oxide, though it still has mild laxative properties. It represents a middle ground between efficacy and tolerability. Other forms such as magnesium glycerophosphate and magnesium lactate are also available in the UK and may be better tolerated by some individuals.

Conversely, magnesium oxide and magnesium hydroxide are less well absorbed, meaning more of the ingested magnesium remains in the gut. These forms are deliberately used in laxative preparations such as milk of magnesia precisely because of their osmotic effects. Magnesium sulphate (Epsom salts) is similarly poorly absorbed and has potent laxative properties. It should not be used as a dietary supplement unless specifically advised by a healthcare professional.

When selecting a magnesium supplement, consider:

• Choosing forms that are better absorbed may reduce gastrointestinal effects

• Checking the elemental magnesium content on the label

• Starting with a lower dose regardless of the form chosen

• Consulting a pharmacist about which formulation best suits your needs

• Seeking medical advice before taking any magnesium supplement if you have kidney problems

• Separating magnesium supplements from certain medications (including tetracycline antibiotics, quinolone antibiotics, bisphosphonates and levothyroxine) by at least 2-4 hours

How to Prevent Diarrhoea When Taking Magnesium

Several practical strategies can help minimise the risk of diarrhoea whilst still obtaining the benefits of magnesium supplementation. The most effective approach is to start with a low dose and increase gradually, allowing your digestive system time to adapt. Begin with 100–150 mg of elemental magnesium daily and increase by small increments every few days, monitoring your bowel response.

Dividing your daily dose throughout the day, rather than taking it all at once, significantly reduces the osmotic load at any given time. For example, if your target dose is 400 mg daily, consider taking 200 mg with breakfast and 200 mg with dinner. This approach distributes the magnesium more evenly and improves overall absorption whilst reducing gastrointestinal side effects.

Taking magnesium with food can slow its transit through the digestive system and enhance absorption, thereby reducing the amount that reaches the colon. Meals containing some fat may further improve absorption of certain magnesium forms. However, avoid taking magnesium supplements with very high-fibre meals, as excessive fibre may interfere with mineral absorption.

Staying well hydrated is important when taking supplements. A normal glass of water (approximately 200 ml) is sufficient to take your supplement.

Additional preventive measures include:

• Switching to a different form if your current supplement causes problems

• Reviewing other medications or supplements that might contribute to loose stools

• Keeping a symptom diary to identify your personal tolerance threshold

• Not exceeding 400 mg/day of supplemental magnesium without medical advice

• Separating magnesium supplements from certain medications (including tetracycline antibiotics, quinolone antibiotics, bisphosphonates and levothyroxine) by at least 2-4 hours

If diarrhoea persists for more than 7 days, or if you experience severe abdominal pain, blood in stools, high fever, or signs of dehydration (dizziness, reduced urination, extreme thirst), contact your GP promptly. Persistent diarrhoea may indicate an underlying condition requiring investigation, and prolonged loose stools can lead to electrolyte imbalances and nutrient malabsorption.

If you suspect an adverse reaction to a magnesium-containing medicine, you can report this through the MHRA Yellow Card scheme.

Why Magnesium Supplements Can Cause Diarrhoea

Understanding the mechanism of action behind magnesium-induced diarrhoea helps explain why this side effect occurs and how to manage it effectively. When magnesium salts are ingested, they dissolve in the stomach and small intestine. Well-absorbed forms are taken up by intestinal cells and enter the bloodstream, where magnesium plays essential roles in over 300 enzymatic reactions, including energy production, protein synthesis, and neuromuscular function.

However, when magnesium intake exceeds the intestinal absorption capacity—or when poorly absorbed forms are used—unabsorbed magnesium remains in the intestinal lumen. This creates an osmotic gradient, drawing water from the surrounding tissues into the bowel. The increased fluid content softens and increases the volume of stool, accelerating intestinal transit time and resulting in loose stools or diarrhoea. This is the same principle exploited therapeutically in osmotic laxatives.

The small intestine has a finite capacity to absorb magnesium, with absorption efficiency decreasing as dose increases. This explains why higher doses are disproportionately more likely to cause diarrhoea—the excess simply cannot be absorbed and exerts osmotic effects.

The UK Reference Nutrient Intake (RNI) for magnesium is 300 mg daily for men and 270 mg daily for women. The guidance level for supplemental magnesium (excluding dietary sources) is 400 mg daily for adults, as recommended by the Expert Group on Vitamins and Minerals and endorsed by the NHS. Exceeding this amount significantly increases the risk of diarrhoea and other gastrointestinal symptoms.

It is worth noting that magnesium from food sources rarely causes diarrhoea because it is present in smaller amounts distributed throughout the day and is accompanied by other nutrients that modulate absorption. Dietary sources include green leafy vegetables, nuts, seeds, whole grains, and legumes. Before starting supplementation, consider whether dietary optimisation might meet your magnesium needs without the gastrointestinal side effects associated with supplements.

People with kidney problems should be particularly cautious with magnesium supplements, as impaired kidney function can lead to magnesium accumulation in the body, potentially causing serious side effects. Always consult your doctor before taking magnesium supplements if you have kidney disease.

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