Does magnesium give you the runs? Yes, magnesium supplements commonly cause diarrhoea, particularly at higher doses or with certain formulations. Whilst magnesium is an essential mineral supporting over 300 bodily functions, supplemental forms can trigger gastrointestinal upset through osmotic effects in the bowel. The likelihood of diarrhoea depends on the dose, the specific magnesium compound used, and individual tolerance. Understanding which forms are most problematic and how to minimise this side effect can help you supplement safely. This article explains why magnesium causes diarrhoea, which types are most likely to cause problems, and practical strategies to reduce gastrointestinal symptoms whilst maintaining supplementation.

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 is an essential mineral involved in over 300 enzymatic reactions in the body, including muscle and nerve function, blood glucose control, and bone health. Whilst dietary magnesium from food sources rarely causes gastrointestinal upset, supplemental magnesium—particularly at higher doses—frequently triggers loose stools or diarrhoea. This is a recognised side effect in the product information for many magnesium preparations.

The threshold at which magnesium causes diarrhoea varies considerably between individuals. Some people experience symptoms at lower doses, whilst others tolerate higher amounts without issue. The NHS advises that adults should not exceed 400 mg of elemental magnesium from supplements per day unless under medical supervision, partly because of this gastrointestinal risk.

It is important to note that whilst diarrhoea from magnesium is generally self-limiting and resolves upon dose reduction or discontinuation, persistent or severe diarrhoea can lead to dehydration and electrolyte imbalances. If you experience bloody stools, severe abdominal pain, signs of dehydration, or high fever, seek urgent medical advice. For diarrhoea that isn't improving after a few days, contact your GP or NHS 111. If you develop diarrhoea while taking magnesium supplements, stop or reduce the dose and reintroduce at a lower dose if appropriate.

Individuals with kidney problems should exercise particular caution with magnesium supplements, as reduced kidney function impairs magnesium excretion and increases the risk of magnesium accumulation.

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 diarrhoea. Understanding these differences can help you select a formulation that meets your needs whilst minimising unwanted effects.

Magnesium oxide and magnesium hydroxide are among the most poorly absorbed forms and consequently the most likely to cause diarrhoea. These inorganic salts have relatively low bioavailability, meaning a significant proportion of the ingested magnesium remains in the intestinal tract. Magnesium hydroxide is the active ingredient in milk of magnesia, which is specifically marketed as a laxative and should not be used as a routine supplement. Magnesium oxide, whilst inexpensive and widely available, frequently causes loose stools even at modest supplemental doses.

Magnesium sulphate (Epsom salts) is another form used therapeutically as a laxative. It should not be used as a routine magnesium supplement due to its potent laxative effect. Magnesium citrate also has laxative properties, particularly at higher doses, making diarrhoea relatively common.

Some evidence and clinical experience suggest that organic forms such as magnesium glycinate and magnesium bisglycinate may be better tolerated by some individuals, though robust comparative studies are limited. When selecting a supplement, it's important to check the specific magnesium compound on the label and note the amount of elemental magnesium provided, as this varies between different magnesium salts and affects dosing.

If you're considering magnesium supplementation, discuss with your pharmacist or GP to determine whether it's necessary and which form might be most appropriate for your specific circumstances.

How to Take Magnesium Without Getting Diarrhoea

Several practical strategies can help minimise the risk of diarrhoea when taking magnesium supplements, allowing you to obtain the benefits of supplementation whilst avoiding gastrointestinal distress.

Start with a low dose and increase gradually. Rather than beginning with the full recommended dose, start with a lower dose and increase gradually as tolerated. This approach allows your digestive system to adapt and helps identify your personal threshold. Remember not to exceed 400 mg of elemental magnesium from supplements daily unless advised by a healthcare professional.

Divide your daily dose throughout the day. Taking smaller amounts with each meal rather than a single large dose reduces the osmotic load in the intestine at any given time. This strategy significantly improves tolerability for many individuals.

Take magnesium with food. Consuming magnesium supplements alongside meals slows gastric emptying and intestinal transit, reducing the concentration of magnesium in the gut and decreasing osmotic effects.

Choose a well-tolerated formulation. As discussed previously, switching from forms like magnesium oxide or citrate to other formulations may help reduce diarrhoea whilst maintaining supplementation.

Stay well hydrated but avoid taking magnesium with very large volumes of water on an empty stomach, as this might accelerate transit through the digestive system.

Be aware of important interactions: Magnesium supplements can reduce the absorption of certain medications, including tetracycline and quinolone antibiotics, bisphosphonates, and levothyroxine. Separate these medications from magnesium supplements by at least 2 hours (check specific product information for guidance).

If diarrhoea persists despite these measures, consult your GP or a registered dietitian to assess whether supplementation is necessary and to explore alternative approaches to optimising magnesium status. Report any suspected side effects from magnesium products via the MHRA Yellow Card Scheme.

Why Magnesium Can Give You the Runs

Understanding the mechanism behind magnesium-induced diarrhoea helps explain why this effect occurs and why certain formulations are more problematic than others.

Osmotic diarrhoea is the primary mechanism. This occurs when poorly absorbed substances remain in the intestinal tract and draw water into the bowel through osmosis. Magnesium salts, particularly those with lower absorption rates, create a hypertonic environment in the gut. Water moves from the intestinal cells and bloodstream into the bowel to dilute this concentrated magnesium solution, resulting in increased stool water content and frequency. This is the same principle that makes magnesium preparations effective as laxatives for constipation.

The degree of osmotic effect depends on the solubility and absorption characteristics of the specific magnesium compound. Highly soluble but poorly absorbed forms (such as magnesium sulphate and magnesium hydroxide) exert the strongest osmotic pull. Forms with higher absorption rates leave less unabsorbed magnesium in the gut, potentially reducing the osmotic effect.

Some research suggests additional mechanisms may contribute to magnesium's laxative effects, though evidence varies in quality. These proposed mechanisms include potential effects on intestinal motility and fluid secretion.

Individual variation in magnesium absorption is substantial and influenced by factors including baseline magnesium status, gut health, age, and concurrent medications. People taking proton pump inhibitors (PPIs) may experience reduced magnesium absorption, which is a separate issue from supplement-induced diarrhoea. The kidneys normally regulate magnesium balance by adjusting urinary excretion, but when oral intake exceeds absorptive capacity, the excess remains in the gut and causes diarrhoea. This self-limiting mechanism actually provides a degree of protection against magnesium toxicity in individuals with normal kidney function.

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