Magnesium Side Effects: Symptoms and How Much Is Too Much

Key points

• Well tolerated: With healthy kidneys, your blood magnesium level stays stable even with a supplement. Whatever your body doesn't need, the kidneys remove.
• 250mg from supplements: Up to this amount, no diarrhoea occurred in studies. Magnesium from food doesn't count towards this.
• Soft stools are the typical sign of too much: They mainly happen with larger amounts taken at once and clear up quickly once you stop. Taking it with food and split doses usually avoids it.
• Count laxatives and antacids too: Many contain magnesium, and it adds up.
• With kidney disease, medication or symptoms: check with a doctor first. Too much magnesium in the blood (hypermagnesaemia) happens almost only with poor kidney function and very high amounts.

Contents

1. Can you overdose on magnesium?
2. How much magnesium is too much?
3. Can you get too much magnesium from food?
4. What are the side effects of magnesium?
5. Magnesium and diarrhoea: why, and what helps?
6. Symptoms of too much magnesium in the blood
7. Who is at risk?
8. Magnesium with kidney disease
9. Magnesium and medication
10. Taken too much magnesium: what to do?
11. How to avoid taking too much
12. When does extra magnesium make sense?
13. How risky is too much magnesium?
14. Frequently asked questions
15. Limits of the research and all sources

Can you overdose on magnesium?

In everyday life, "too much" almost always means something harmless. If you take more magnesium than your gut can absorb, the rest stays in the gut. It draws in water, and your stool becomes soft. That's uncomfortable, but harmless, and it passes once you stop. A smaller amount taken with food usually settles it.

A second, rare form affects the blood. Doctors call this hypermagnesaemia, meaning too high a magnesium level in the blood. This happens almost only when the kidneys can no longer remove excess magnesium and a lot of it arrives at the same time.

What the studies show: The EU authority describes diarrhoea as the most sensitive adverse effect of magnesium from supplements. It is mild, temporary and reversible, and the gut often adjusts within a few days.¹ Kidney specialists, on the other hand, see severe hypermagnesaemia almost exclusively in people with significantly reduced kidney function combined with a lot of magnesium from supplements, laxatives or antacids.⁶

How much magnesium is too much?

The key figure is 250mg of magnesium a day from supplements. That's the EU's tolerable upper intake level for adults, pregnant and breastfeeding women, and children from age 4.¹ For children aged 1 to 3, there is no set value; magnesium supplements should only be given in consultation with a paediatrician.

This limit isn't a toxic threshold. It marks the amount up to which no diarrhoea was observed in studies. The US sets its limit at 350mg for the same reason.³

Amount of magnesium from supplements per day	What is known
up to 250mg	no diarrhoea in studies, EFSA upper limit1
around 360mg	diarrhoea in a small proportion of adults1
350mg	upper limit in the US3
over 2,500mg at once	amount usually exceeded in the few reported poisoning cases, almost always from laxatives or antacids1

How much magnesium you need overall is a different question. The German Society for Nutrition (DGE) sets the figure for adults at 300mg (women) and 350mg (men) a day, from all sources combined.² More on this in How much magnesium per day?

What the studies show: The EU experts found no laxative effect in the studies they reviewed up to 250mg a day. From around 360mg, mild diarrhoea occurred in a small proportion of adults.¹ A more recent analysis takes a more generous view: in seven out of ten studies with 128 to 1,200mg a day, magnesium was no more likely to cause diarrhoea than a placebo.⁴ What that means for you is covered in our assessment.

Can you get too much magnesium from food?

Practically not. The 250mg limit applies only to easily soluble magnesium salts in supplements, water and fortified foods. Magnesium from nuts, wholegrains or vegetables explicitly doesn't count.¹ There, it's largely bound, for example to phytic acid, and considered poorly soluble.¹

Mineral water rich in magnesium is different: the magnesium in it is dissolved and counts like a supplement.¹ If you drink a lot of it every day and also take capsules, add both together. The amount is on the bottle label in mg per litre.

What the studies show: The EU experts deliberately excluded magnesium from food from their assessment. The studies behind the upper limit tested supplements on top of a normal diet.¹ On top of that, healthy kidneys reliably remove excess magnesium.⁶

What are the side effects of magnesium?

At usual amounts, hardly any. That also applies to daily use over months: there is no time limit up to 250mg a day.¹ When magnesium does cause symptoms, it's usually soft stools or diarrhoea, sometimes with a rumbling stomach.

Serious effects such as weakness, drowsiness or low blood pressure belong to hypermagnesaemia. They only occur at significantly raised blood levels, which are hard to reach through supplements with healthy kidneys.^{6, 7}

What the studies show: An analysis of 48 controlled studies with more than 2,100 participants shows how little the blood level moves in healthy people. With 197 to 994mg of magnesium a day, it rose on average by only 0.04 mmol/L. The normal range is 0.7 to 1.0 mmol/L.^{5, 7} The kidneys remove the rest through urine.⁵

Magnesium and diarrhoea: why, and what helps?

Magnesium that isn't absorbed binds water in the gut. This makes your stool softer and bulkier. Laxatives with magnesium oxide, hydroxide or Epsom salt (magnesium sulfate) use exactly this effect.^{1, 11} In supplements, it's unwanted, but the mechanism is the same.

Which magnesium doesn't cause diarrhoea? The amount you take at once matters most. Epsom salt has a particularly strong laxative effect, because the sulfate also holds water in the gut.¹ Magnesium oxide is used as a laxative.¹¹ Magnesium glycinate was better tolerated than magnesium oxide in one small study.¹³ More in our article on magnesium glycinate.

How long does the diarrhoea last? Only as long as unabsorbed magnesium is in your gut. It settles once you stop.¹

What if you already have diarrhoea? Severe or prolonged diarrhoea can itself cost you magnesium. In that case, avoid extra high amounts and get the cause checked by a doctor instead.

What the studies show: The EU authority notes that a daily amount is better tolerated when split across several portions.¹ That magnesium-related diarrhoea often goes unnoticed is shown by an older study of 359 people with chronic diarrhoea. In 15 of them (4.2%), too much magnesium from antacids or supplements was an important cause.¹²

Symptoms of too much magnesium in the blood

The good news first: supplements raise the blood level in healthy people by only 0.04 mmol/L on average.⁵ That's far from the ranges in the table below. Hypermagnesaemia is a condition diagnosed in hospital. The early signs are non-specific, so it's easily missed.⁷ Slightly raised levels often cause no symptoms at all. The higher the level, the clearer the symptoms:

Magnesium in the blood	Typical signs, per review
0.7 to 1.0 mmol/L	normal range
up to 2.2 mmol/L	often no symptoms
2.2 to 3.5 mmol/L	nausea, dizziness, weakness, confusion
over 3.5 mmol/L	drowsiness, weakened reflexes, headache, flushed skin, urination problems, blurred vision, a mild drop in blood pressure
very high levels, over 6.5 mmol/L (almost only with severely impaired kidney function plus a lot of magnesium)	life-threatening, emergency

Values based on Aalhamad et al. 2023.⁷ Normal ranges vary slightly by laboratory.

What the studies show: In a Japanese hospital, 15 people with severe hypermagnesaemia needed emergency dialysis. All had taken magnesium oxide as a laxative, and 14 of them were over 65. The most common symptoms were impaired consciousness, low blood pressure, a slow heartbeat and breathing problems.⁹

Who is at risk?

For most people, a real overdose isn't an issue. The main cause is reduced kidney function. The kidneys regulate your magnesium balance. If they're working poorly, magnesium stays in the body.^{6, 8} The second cause is large amounts, usually from laxatives or antacids, not from typical supplements.^{1, 6}

Extra care is worthwhile for:

• People with kidney disease, especially advanced kidney disease.^{6, 8}
• Older people who take magnesium-based laxatives daily, especially for persistent constipation. The longer the product stays in the gut, the more gets absorbed.^{7, 9}
• People with inflammatory bowel disease, because an inflamed gut can absorb more magnesium.⁷
• Rarely, an underactive thyroid, other hormone disorders or lithium also play a role, because they slow removal through the kidneys.⁷

Old age alone isn't a risk factor. In an observation of 64 people with an average age of 81, who took around 680mg of magnesium a day as a laxative, no hypermagnesaemia occurred.¹

What the studies show: A hospital study followed 320 people prescribed magnesium oxide. The risk of raised blood levels increased with weaker kidney function, from 1,650mg of magnesium oxide a day, and after more than five weeks of use.¹⁰ 1,650mg of magnesium oxide is roughly 1,000mg of magnesium, four times the EFSA upper limit (our calculation: magnesium makes up 60% of the mass of magnesium oxide).

Magnesium with kidney disease

If you have kidney disease, only take magnesium after talking to your doctor. That also applies to magnesium-based laxatives and antacids. This is where the real risk of an overdose lies.^{6, 8}

That doesn't mean magnesium is always off-limits with kidney disease. Many people with kidney disease actually have too little of it. Whether and how much makes sense can only be decided with blood tests and medical supervision.

What the studies show: One review concludes that magnesium was given safely in studies for kidney disease, without severe hypermagnesaemia.¹⁴ A controlled study of 148 people with advanced kidney disease, however, found downsides. With magnesium, there were significantly more gastrointestinal complaints (35 versus 9) and more serious adverse events than with a placebo.¹⁵

Magnesium and medication

Magnesium can slow the absorption of some medicines if you take both at the same time. This isn't an overdose, but it's a common reason people search for "side effects". A gap of a few hours usually avoids it.

If you take …	… then note
Thyroid hormone (levothyroxine)	take at separate times, agree the gap with your GP or pharmacist16
Antibiotics (tetracyclines, quinolones)	magnesium 4 hours before or 2 hours after17
Magnesium-based laxatives or antacids	count towards your daily amount, don't dose extra on top1, 6
Lithium	can slow the removal of magnesium, discuss with your doctor7
Water tablets and acid-reducing tablets	tend to lower your magnesium level18

The detailed table with further medicines is in What should you not take with magnesium?

Taken too much magnesium: what to do?

That depends on what happened:

• You have soft stools or diarrhoea: stop magnesium or reduce the amount, drink enough. This usually settles within a few days.¹ Start again afterwards with smaller amounts taken with food.
• You took clearly more than planned once, but feel fine: skip the next few doses. With healthy kidneys, the excess is removed.⁶ If you're unsure, ask at a pharmacy.
• You have kidney disease or took large amounts of laxatives: contact your doctor, or NHS 111 for advice.

What the studies show: Reviews stress that the signs are non-specific. That's why diagnosis is often made late, especially in older people who struggle to describe their symptoms.^{7, 9} Caught in time, it can usually be treated well.⁷

How to avoid taking too much

A few simple rules keep you on the safe side:

1. Add up everything that supplies dissolved magnesium: capsules, powder, effervescent tablets, magnesium-rich mineral water, laxatives and antacids. The total shouldn't exceed 250mg without medical advice.¹
2. Check the "magnesium" figure on the label, not the weight of the compound. 1,000mg of magnesium citrate contains noticeably less than 1,000mg of magnesium.
3. Take magnesium with food and split larger amounts.¹
4. Don't take several magnesium products at once, such as a mineral supplement plus separate magnesium.
5. Check with your doctor beforehand if you have kidney disease or take medication long-term.⁸

When does extra magnesium make sense?

Extra magnesium can make sense if you get less from food than you need. The estimated figure is 300mg for women and 350mg for men a day.² A diet low in wholegrains, pulses, nuts and green vegetables makes that harder.

Some medicines lower your magnesium level, for example water tablets and acid-reducing tablets (proton pump inhibitors).¹⁸ If you take these long term, discuss your magnesium level with your GP. If you train a lot, your needs are a little higher too.

Magnesium contributes to normal muscle function. Magnesium contributes to a reduction of tiredness and fatigue. How much you need yourself, and how to estimate it, is explained in How much magnesium per day? Which form suits you is covered in Which magnesium is best?

How risky is too much magnesium?

For healthy adults, magnesium from supplements is well tolerated. If you stay at 250mg or less, current evidence suggests you need to expect neither diarrhoea nor unusual blood levels.¹ Take a bit more, and soft stools are the worst you're likely to get. It only gets serious with poor kidney function, or with large amounts from laxatives and antacids.^{6, 7}

Frequently asked questions

How much magnesium a day?

The German Society for Nutrition gives an estimated 300mg for women and 350mg for men a day, from food and supplements combined.² From supplements alone, it should be no more than 250mg.¹ More detail in How much magnesium per day?

What are the side effects of taking magnesium daily?

Usually none, at typical amounts. The typical side effect is soft stools or diarrhoea, especially with larger amounts taken at once.¹ The same applies to magnesium at night.

Is 400mg of magnesium too much?

As a total from food and supplements, no, that's within the range of daily needs.² As a supplement alone, 400mg is above the EFSA upper limit of 250mg and above the US figure of 350mg.^{1, 3} With healthy kidneys, that's not dangerous, but diarrhoea becomes more likely. If your doctor recommends this amount, split it across two meals.

Is 500mg of magnesium too much?

From a supplement, that's double the EFSA upper limit.¹ For healthy people, it's not a poisoning risk, but diarrhoea is clearly more likely. Without a medical reason, we'd advise against it. With kidney disease: only after checking with your doctor.⁸

What symptoms does too much magnesium cause?

At first, usually soft stools or diarrhoea.¹ Only at significantly raised blood levels, which occur almost only with poor kidney function, do nausea, dizziness, weakness, drowsiness, low blood pressure and a slow pulse follow.⁷ Symptoms like these should be checked by a doctor straight away.

What are the main causes of a magnesium overdose?

Reduced kidney function and very large amounts of magnesium, usually from laxatives or antacids.^{6, 8} Typical supplements alone are rarely the reason with healthy kidneys.¹

Can too much magnesium cause tingling?

Tingling isn't among the described signs of too much magnesium.⁷ Reviews link muscle cramps and overexcitable nerves and muscles more with a magnesium deficiency instead.¹⁹ Tingling can have many causes, though. If it persists, get it checked by a doctor.

Does too much magnesium cause frequent urination?

That's not a typical sign. At very high blood levels, the literature describes urination problems instead, because the bladder muscle relaxes.⁷ Frequent urination usually has other causes and should be checked by a doctor if it lasts.

Does too much magnesium harm your liver?

There's no evidence for that. Magnesium is removed through the kidneys, not broken down by the liver.⁶ In an analysis of seven studies, liver values didn't rise with magnesium.²⁰

Can magnesium affect your thyroid?

Magnesium can slightly reduce the absorption of thyroid hormone (levothyroxine) if you take both at the same time. So take them at separate times.¹⁶ The reverse can also happen: a severe underactive thyroid can, in rare cases, slow the removal of magnesium.⁷

Limits of the research and all sources

Evidence on hypermagnesaemia comes mainly from case reports and hospital studies, mostly from Japan, where magnesium oxide is often prescribed as a laxative. They almost always involve ill or older people and much higher amounts than in supplements. Studies on side effects often record diarrhoea only by self-report, without a consistent definition. How quickly diarrhoea starts after taking a supplement hasn't been studied precisely. The EFSA upper limit dates from 2001, and there has been no recent EU reassessment so far. The more recent analysis that sees more room was funded by a magnesium research association, and one author holds a patent on a magnesium product. An older study it cites, with 50 adults and magnesium oxide, also shows that diarrhoea can be quite common at higher amounts.⁴ For kidney disease, the studies are inconsistent, and whether magnesium caused the serious events observed there remains open.^{14, 15}

Show all 20 studies and sources

#	Study	Who was studied	Result	Strength of evidence
1	EFSA / SCF 2001	official assessment	upper limit of 250mg/day from supplements, water, fortified foods; food excluded; diarrhoea from approx. 360mg in a few	official assessment
2	DGE 2021	reference values	300mg (women), 350mg (men) per day	official recommendation
3	Institute of Medicine 1997	US reference values	upper limit of 350mg/day from supplements	official assessment
4	Costello et al. 2023, Adv Nutr	10 studies, 1997 to 2022	7 of 10 studies: diarrhoea no more common than with placebo	review with assessment, funded by a magnesium association
5	Zhang et al. 2016, J Nutr	48 studies, 2,131 people	blood level rose by an average of 0.04 mmol/L with supplements	meta-analysis
6	Van Laecke 2018, Acta Clin Belg	review	severe hypermagnesaemia almost only with poor kidney function plus a lot of magnesium	review
7	Aalhamad et al. 2023, Medicina	review	normal range, symptoms by blood level, causes	review
8	Rosner et al. 2023, Mayo Clin Proc	review	hypermagnesaemia mainly with reduced kidney function and magnesium medicines	review
9	Nishikawa et al. 2018, Acute Med Surg	15 hospital patients needing emergency dialysis	all had taken magnesium oxide, 14 over 65	case series
10	Wakai et al. 2019, J Pharm Health Care Sci	320 hospital patients on magnesium oxide	risk rises with weaker kidney function, from 1,650mg/day, after more than 36 days	observational study
11	Mori et al. 2021, Nutrients	review	magnesium oxide has a laxative effect because it holds water in the gut	review
12	Fine et al. 1991, N Engl J Med	359 people with chronic diarrhoea	too much magnesium was an important cause in 4.2%	diagnostic study
13	Schuette et al. 1994, JPEN	12 patients after bowel surgery	glycinate better tolerated than magnesium oxide	small controlled study
14	Vermeulen et al. 2023, Nutrients	review, kidney disease	magnesium given safely in studies, no severe hypermagnesaemia	review
15	Bressendorff et al. 2023, J Am Soc Nephrol	148 people with advanced kidney disease, 12 months	more gastrointestinal complaints and serious adverse events with magnesium	controlled study
16	Attinger et al. 2025, Clin Transl Sci	15 healthy people	thyroid hormone: up to 12% less absorption	small
17	Ogawa & Echizen 2011, Drugs	review	antibiotics: 4 hours before or 2 hours after	review
18	Gröber 2019, Int J Mol Sci	review	diuretics and acid-reducing tablets lower magnesium	review
19	Bosman et al. 2021, Proc Nutr Soc	review	magnesium deficiency: muscle cramps, tiredness, among others	review
20	Leyva-Velázquez et al. 2026, Nutr Metab Cardiovasc Dis	7 studies	liver values didn't rise with magnesium	meta-analysis

Author: PRONESIUM Editorial Team · Last reviewed: 30/09/2026 · This article does not replace medical advice.