Magnesium side effects, and who should take care

What are the side effects of magnesium?

A good rule of thumb is that magnesium's side effects rise with the dose, and most of them are felt in the gut. Diarrhoea, loose stools, stomach pain and wind are the usual ones. The NHS says 400 mg or less a day of magnesium from supplements is unlikely to cause any harm, but more can cause diarrhoea [1].

In a Cochrane review of 11 trials in people with leg cramps, stomach side effects, mainly diarrhoea, affected 11% to 37% of people taking magnesium. On a placebo, the figure was 10% to 14% [2].

Magnesium that isn't absorbed draws water into the bowel, which is why it loosens stools [4]. This guide is about magnesium you take by mouth.

How much magnesium is too much?

From supplements, more than 400 mg of magnesium a day. That's the NHS line, and it covers all your supplements together, multivitamins included. There isn't enough evidence on what high doses do over a long time [1].

So is 500 mg too much? As 500 mg of magnesium, yes, because it's above that line. As 500 mg of magnesium glycinate, it gives only about 70 mg of magnesium. That's because glycinate is only about 14% magnesium by weight [5].

Go by the magnesium figure on the label. Our guide to how much magnesium per day shows how to read it.

Can magnesium cause diarrhoea?

Yes. It's the most common side effect [2], and it's more likely as the dose goes up [6]. In a 12-week trial of 59 adults aged 55 and over, half of those taking 400 mg of magnesium oxide noticed looser stools or other gut changes. On a placebo, it was 7% [3].

For several people, the diarrhoea lasted only for the first 2 days. Just 1 of the 29 people taking magnesium stopped because of side effects [3].

If it happens, UK magnesium medicine leaflets advise cutting the dose, then building up again slowly if needed [6]. Taking smaller amounts through the day is another option, and it's how UK magnesium medicines are taken [6]. In practice, that could mean splitting your capsules between morning and evening rather than taking them all at once.

What are the side effects of magnesium glycinate?

Few, going by the main trial in healthy adults. In a 2025 trial, 153 adults who slept poorly took 250 mg of magnesium a day as bisglycinate, or a placebo, for 4 weeks. There were 2 side effects on magnesium and 7 on the placebo, and nobody stopped taking magnesium because of one [5].

No trial in healthy adults has compared glycinate's side effects with other forms over weeks. The one long trial of different forms didn't include it. In that 24-week trial, citrate caused no more stomach symptoms than a placebo, oxide caused fewer than citrate, and sulfate caused more than a placebo [4]. A magnesium maker helped fund it.

Read more in our guides to magnesium glycinate, citrate and malate.

Does magnesium make you sleepy?

Not during the day. In the 2025 glycinate trial, people took it before bed for 4 weeks. Their daytime sleepiness and tiredness were no different from those on a placebo Early [5].

UK magnesium medicines have no or negligible effect on driving [6][7]. One leaflet lists tiredness with high doses taken long term as very rare, in fewer than 1 in 10,000 people. It says this may mean your blood magnesium has risen [7].

Drowsiness is also a sign of too much magnesium in the blood, which is rare, as the section below explains. For whether magnesium helps you sleep at night, see our guide to magnesium for sleep.

Can magnesium cause constipation?

It's unlikely, since magnesium usually has the opposite effect. It tends to loosen stools rather than firm them, and trials report diarrhoea-type side effects, not constipation [2][3]. In Japan, magnesium oxide is widely used as a laxative [8].

If you feel constipated while taking a supplement, check its other ingredients and ask a pharmacist.

Can you take magnesium with blood pressure tablets?

Usually, yes, but tell your GP or pharmacist when you start. Magnesium can lower blood pressure a little. Across 38 trials, it lowered the top number by about 3 mmHg on average. In people already taking blood pressure medicines, it lowered it by nearly 8 mmHg Moderate [9].

Some medicines lower your magnesium over time. These include water tablets, such as furosemide, and long-term use of heartburn tablets called PPIs, such as omeprazole [6][7].

Magnesium can also stop some medicines being absorbed. UK leaflets advise leaving 2 to 3 hours between magnesium and other medicines where you can, and 3 to 4 hours for iron and bone medicines such as alendronic acid [6]. Our guide on when to take magnesium lists the gap for each one, including antibiotics and levothyroxine.

Is it safe to take magnesium every day?

Yes, for most adults, at 400 mg a day or less from supplements [1]. Trials that gave magnesium every day for 12 and 24 weeks found mainly stomach side effects [3][4].

Ask your GP first if you have kidney disease. Healthy kidneys clear extra magnesium, but UK magnesium medicines aren't given to people with severe kidney disease [6][7]. One leaflet also advises care for people with heart rhythm problems [7].

What happens if you take too much magnesium?

Usually loose stools or diarrhoea [6]. Too much magnesium in the blood, called hypermagnesaemia, is rare. A UK leaflet says that with healthy kidneys, magnesium poisoning from tablets isn't expected [7].

It mostly happens when the kidneys can't clear the extra, or with large doses of magnesium laxatives or antacids [6][8]. Japan's medicines regulator reviewed 29 reported cases of too much magnesium in people taking magnesium oxide between 2012 and 2015, 4 of them fatal. About 10 million people were treated with it there in 2013 [8].

Most were aged 65 or over and taking it for constipation, and some had kidney problems [8]. These were mostly laxative users, not people taking a daily supplement. Signs include feeling or being sick, flushing, thirst, drowsiness, confusion, muscle weakness, low blood pressure and a slow heartbeat [6][8].

Where Nooroots fits

Nooroots Triple Magnesium gives you 301 mg of magnesium a day, 80% of the NRV, from glycinate, citrate and malate in three capsules. That's below the NHS's 400 mg line for supplements. Take them together or spread them through the day, whichever suits you.

Triple Magnesium's magnesium contributes to a reduction of tiredness and fatigue and to normal psychological function.

Coming soon. Triple Magnesium isn't on sale yet.

Frequently asked questions

Can magnesium cause stomach pain?

It can. In the 24-week trial of three forms, stomach pain was one of the most common side effects, along with mild diarrhoea and wind [4]. A UK leaflet advises stopping for a while, then starting again at a lower dose [7].

Can magnesium cause headaches?

None of the trials here found headaches more often on magnesium than on a placebo. In the 2025 glycinate trial, one person on the placebo had a headache and nobody taking magnesium did [5].

Can magnesium keep you awake?

Trials haven't found that it does. In the 2025 glycinate trial, one person on the placebo reported disturbed sleep and nobody taking magnesium did [5].

Can magnesium cause heart palpitations?

There's little research, but nothing so far suggests it. In a 12-week trial, magnesium didn't change the number of extra heartbeats recorded on a 2-week heart monitor Early [3]. If you notice palpitations, speak to your GP.

How do you get rid of excess magnesium?

Stop taking it. Healthy kidneys clear the extra, and stopping is all most mild cases need, while severe cases are treated in hospital [6]. If you have any of the signs above, get medical help straight away.

Can you take magnesium while pregnant?

Ask your midwife, GP or pharmacist first, and the same goes while breastfeeding. The NHS advises folic acid and vitamin D supplements, and says a varied diet gives you most other vitamins and minerals [10].

What are the side effects of magnesium malate?

We found no trial that reports them for malate on its own. Expect the same as other forms, mainly loose stools at higher doses [1]. Our magnesium malate guide has more.

References

Every study here is listed so you can read it yourself. This guide is for general information and doesn't replace advice from your GP or pharmacist. We review it when new research is published.

1. NHS. Vitamins and minerals: others (magnesium section). Accessed 17 October 2026. nhs.uk/conditions/vitamins-and-minerals/others
2. Garrison SR et al. (2020). Magnesium for skeletal muscle cramps. Cochrane Database of Systematic Reviews 9: CD009402. PMID 32956536
3. Lutsey PL et al. (2018). A pilot randomized trial of oral magnesium supplementation on supraventricular arrhythmias. Nutrients 10(7): 884. PMID 29996476
4. Schutten JC et al. (2022). Effects of magnesium citrate, magnesium oxide, and magnesium sulfate supplementation on arterial stiffness: a randomized, double-blind, placebo-controlled intervention trial. Journal of the American Heart Association 11(6): e021783. PMID 35253448
5. Schuster J et al. (2025). Magnesium bisglycinate supplementation in healthy adults reporting poor sleep: a randomized, placebo-controlled trial. Nature and Science of Sleep 17: 2027–2040. PMID 40918053
6. Neoceuticals Ltd. Neomag 4 mmol chewable tablets: Summary of Product Characteristics (sections 4.2, 4.3, 4.5, 4.7, 4.8 and 4.9). Electronic medicines compendium, updated 28 September 2026. medicines.org.uk/emc/product/2678/smpc
7. Kora Healthcare. Magnesium Kora Healthcare 4 mmol (97 mg) tablets: Summary of Product Characteristics (sections 4.3, 4.4, 4.5, 4.7, 4.8 and 4.9). Electronic medicines compendium, updated 7 November 2025. medicines.org.uk/emc/product/12118/smpc
8. Pharmaceuticals and Medical Devices Agency, Japan (2015). Hypermagnesaemia caused by magnesium oxide. Pharmaceuticals and Medical Devices Safety Information No. 328. pmda.go.jp (PDF)
9. Argeros Z et al. (2025). Magnesium supplementation and blood pressure: a systematic review and meta-analysis of randomized controlled trials. Hypertension 82(11): 1844–1856. PMID 41000008
10. NHS. Vitamins and supplements in pregnancy. Best Start in Life. Accessed 17 October 2026. nhs.uk/best-start-in-life