Does magnesium give you the runs? Yes, magnesium supplements commonly cause diarrhoea, particularly at higher doses or with certain formulations. This gastrointestinal side effect occurs because unabsorbed magnesium draws water into the bowel, triggering loose stools. The likelihood and severity depend on the dose, the specific form of magnesium, and individual tolerance. Whilst dietary magnesium from food rarely causes digestive upset, supplemental forms frequently affect bowel movements. Understanding which types of magnesium are most problematic and how to minimise this side effect can help you supplement safely and effectively.

Does Magnesium Cause 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 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 digestive upset, supplemental forms—particularly at higher doses—frequently trigger loose stools or diarrhoea.

The threshold for developing diarrhoea varies considerably between individuals. Some people experience digestive upset with doses as low as 200–300 mg of elemental magnesium, whilst others tolerate higher amounts without difficulty. The NHS advises that adults should not exceed 400 mg of magnesium from supplements daily, as higher doses increase the risk of adverse effects, with diarrhoea being the primary dose-limiting factor.

People with kidney problems should be particularly cautious with magnesium supplements, as reduced kidney function can lead to magnesium accumulation. Additionally, magnesium supplements can interact with certain medications, including levothyroxine, tetracycline and quinolone antibiotics, and bisphosphonates. These medications should be taken at least 4 hours apart from magnesium supplements.

Magnesium-induced diarrhoea is generally self-limiting and resolves once supplementation is reduced or discontinued. However, persistent or severe diarrhoea can lead to dehydration and electrolyte imbalances, particularly in vulnerable populations such as older adults. Seek medical advice if you experience diarrhoea that lasts more than 48 hours, contains blood, is accompanied by severe abdominal pain or fever, or if you develop signs of dehydration. Call NHS 111 if you're unsure what to do.

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 can influence both its absorption characteristics and its propensity to cause diarrhoea. Understanding these differences may help you select a formulation that meets your needs whilst minimising digestive upset.

Magnesium oxide and magnesium hydroxide are among the forms that may be more likely to cause diarrhoea. These inorganic salts tend to have lower absorption rates, meaning that more magnesium remains in the intestinal tract, where it draws water into the bowel through osmotic action. Magnesium hydroxide is the active ingredient in milk of magnesia, which is specifically marketed as a laxative, illustrating its effect on bowel motility.

Magnesium citrate also has notable laxative properties. In the UK, magnesium citrate (often combined with sodium picosulfate in products like Picolax) is commonly used in bowel cleansing preparations before colonoscopies and other medical procedures precisely because of its reliable effect on bowel movements.

Other forms of magnesium include magnesium glycinate, magnesium malate, and magnesium taurate. Some evidence suggests these forms may be better tolerated by some individuals, though research directly comparing the digestive effects of different magnesium formulations is limited. Individual responses vary considerably, and the dose taken remains a critical factor regardless of the form used.

When selecting a magnesium supplement, checking the specific form listed on the label and starting with a lower dose can help you assess your personal tolerance. If you have a sensitive digestive system, discuss with your pharmacist or GP which magnesium formulation might be most suitable for your specific health needs.

How to Take Magnesium Without Getting Diarrhoea

Several practical strategies can help minimise the risk of magnesium-induced diarrhoea whilst still obtaining the benefits of supplementation. Implementing these approaches systematically often allows individuals to find their optimal dose and formulation.

Start with a low dose and increase gradually. Rather than beginning with the full recommended dose, start with 100–150 mg of elemental magnesium daily and increase by small increments every few days as tolerated. This titration approach allows your digestive system to adapt and helps identify your personal threshold for gastrointestinal symptoms. Remember not to exceed the NHS recommended upper limit of 400 mg per day from supplements.

Divide your daily dose throughout the day. Taking smaller amounts of magnesium with each meal—rather than a single large dose—reduces the concentration of magnesium in the intestine at any given time, thereby decreasing osmotic load and the likelihood of diarrhoea. For example, if your target dose is 300 mg daily, consider taking 100 mg with breakfast, lunch, and dinner.

Consider different formulations. If one form of magnesium causes digestive upset, another formulation might be better tolerated. Discuss options with your pharmacist or GP if you're experiencing persistent digestive issues.

Take magnesium with food. Consuming magnesium supplements alongside meals slows gastric emptying and may reduce the osmotic effect in the small intestine.

Be aware of important medication interactions. If you take prescription medications, timing is important. Separate magnesium supplements from levothyroxine, tetracycline or quinolone antibiotics, and bisphosphonates by at least 4 hours.

Seek medical advice before supplementing if you have kidney problems, are pregnant or breastfeeding, or have a medical condition. If digestive symptoms persist despite these strategies, consult your GP or a registered dietitian for personalised guidance.

Why Magnesium Can Give You the Runs

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

Osmotic diarrhoea is the principal mechanism. When magnesium is not fully absorbed in the small intestine, it remains in the intestinal tract where it creates an osmotic gradient that draws water from the surrounding tissues into the bowel. This increased fluid volume stimulates intestinal motility and reduces the time available for water reabsorption in the colon, resulting in loose, watery stools. The effect is dose-dependent: higher concentrations of unabsorbed magnesium produce more pronounced osmotic effects and more severe diarrhoea.

Individual variation in absorption capacity plays a significant role. The small intestine has a limited capacity to absorb magnesium, and this absorption efficiency can vary based on dose, dietary factors, and individual differences. Generally, absorption efficiency decreases as the dose increases, meaning that larger single doses result in proportionally more unabsorbed magnesium remaining in the gut.

Certain medical conditions can increase susceptibility. Conditions including inflammatory bowel disease, coeliac disease, and chronic diarrhoea from other causes can impair magnesium absorption, making affected individuals more susceptible to magnesium-induced diarrhoea.

If you experience persistent gastrointestinal symptoms while taking magnesium supplements, it's important to consult your GP. They may recommend adjusting your dosage, changing formulations, or investigating potential underlying conditions. If you suspect an adverse reaction to a magnesium supplement, you can report it through the MHRA Yellow Card Scheme, which helps monitor the safety of medicines and supplements in the UK.

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