Magnesium Side Effects: Diarrhoea, Nausea, Headaches, What to Do

The common side effect of a magnesium supplement is loose stools, because magnesium that is not absorbed stays in the bowel and draws water in. In the trials tabulated by the European food safety committee it began in a few people at about 365 mg a day and in nobody at 250 mg, it settled within days, and it was less likely when the daily dose was split.¹ Nausea and stomach irritation come next; serious excess is uncommon and seen mainly with impaired kidneys or magnesium-containing medicines.² This page sits inside the Wellgard vitamins and minerals guide.

Why does magnesium cause diarrhoea, and at what dose?

The European committee that set the supplement limit in 2001 found no mild diarrhoea in the trials at 250 mg a day and the first cases at about 365 mg; it noted that the bowel adapts within days and that "a given daily dose of Mg is better tolerated when it is divided into several portions".¹

The Cochrane review of magnesium for cramps, 11 trials and 735 people, found "mostly gastrointestinal adverse events (e.g. diarrhoea)" in 11% to 37% of participants against 10% to 14% on placebo; minor adverse events were 1.51 times placebo (95% CI 0.98 to 2.33), not statistically significant, from low-certainty evidence.³ The NHS vitamins and minerals page (reviewed 3 August 2020) says "Taking high doses of magnesium (more than 400mg) for a short time can cause diarrhoea" and that 400 mg or less a day from supplements "is unlikely to cause any harm".⁴ The 400 mg is the UK Expert Group on Vitamins and Minerals' guidance level, "For guidance purposes only".⁷

Does the form of magnesium change the side effects?

One 2024 supplier-funded single-dose crossover in 40 healthy adults compared oxide, citrate and bisglycinate at the same elemental dose on self-reported bowel effects; no trial of weeks has compared their loose-stool rates. In 12 people with part of the small bowel removed, a 100 mg dose of a glycine chelate was "better tolerated by all patients" than oxide, with overall absorption the same.⁸ In 80 pregnant women, 300 mg a day of magnesium bisglycinate for four weeks caused "no significant differences" in nausea or diarrhoea compared with placebo.⁹ The 2024 crossover reported less "intestinal motility" and "gastric heaviness" with the funder's microencapsulated ingredient than with oxide, citrate or bisglycinate.¹⁰ The forms are compared study by study in which magnesium is best, what magnesium glycinate is and what magnesium citrate is.

Can magnesium cause nausea, headaches or tiredness?

Yes, but rarely in the trials. In a pilot of 29 women with breast cancer taking magnesium oxide at 400 mg rising to 800 mg a day, two stopped, one with headache and one with nausea, and two had mild (grade 1) diarrhoea.¹¹ The 600 mg migraine trial recorded gastric irritation in 4.7%.⁵ The prescription magnesium monograph lists diarrhoea and soft stools as uncommon, fatigue and high blood magnesium as rare or very rare, and gastrointestinal irritation at a frequency not known.¹² For nausea, take it with a meal; see when to take magnesium.

Can magnesium cause vivid dreams, palpitations or make you feel wired?

No trial abstract we read records vivid dreams, nightmares, palpitations or dizziness as a side effect of oral magnesium; they are forum reports. Palpitations belong to a different picture: very low magnesium and dangerously high blood magnesium both affect the heart, the latter seen mainly with kidney impairment.² The "wired after magnesium" reports are taken up on does magnesium help you sleep, and the signs of low magnesium covers the deficiency side.

Can magnesium cause constipation?

No trial reports constipation as a side effect of any magnesium salt; the 289-woman hot-flush trial recorded a "lower incidence of constipation" on 800 to 1,200 mg of oxide.⁶ Magnesium does appear in bowel medicines, at medicine doses: magnesium hydroxide is licensed for constipation, and magnesium citrate with sodium picosulfate for bowel evacuation before radiological examination, endoscopy or surgery.¹³ In a 90-patient trial of chronic constipation, 1.5 g a day of magnesium oxide for 28 days improved symptoms in 68.3% against 11.7% on placebo, with no severe treatment-related adverse events.¹⁴ The NHS constipation page (reviewed 26 October 2023) says "Most laxatives work within 3 days. They should only be used for a short time."¹⁵ For constipation itself, start with the Wellgard constipation guide.

What happens if you take too much magnesium?

Hypermagnesaemia, too much magnesium in the blood, is "relatively uncommon but potentially life-threatening"; the high-risk groups are "patients with impaired renal function or those receiving magnesium-containing medications or supplements", and hospital management means stopping magnesium, intravenous fluids or, in severe cases, dialysis.² The prescription monograph gives disorders of cardiac conduction as a reason not to take magnesium.¹² Healthy kidneys handle a surplus, reabsorbing 95% of filtered magnesium and excreting the rest in urine, so stopping the supplement is the step that matters, and drinking extra water is not the mechanism.⁷ The limits are set out in is it OK to take magnesium every day and the daily amounts in how much magnesium per day.

Which medicines make magnesium side effects more likely, or lower your magnesium?

Two directions. Some medicines lower magnesium: a 2021 review names "antibiotics (e.g. aminoglycosides, amphotericin B), diuretics, antineoplastic drugs (cisplatin and cetuximab), calcineurin inhibitors, and proton pump inhibitors".¹⁶ Antacids and laxatives that contain magnesium add to the total, which is why they appear in the hypermagnesaemia risk group.² The NHS alendronic acid page (reviewed 7 May 2025) says that if you need another medicine by mouth, "wait for at least 30 minutes", and lists "laxatives containing magnesium" among the medicines to tell your doctor about.¹⁷ No NHS medicine page gives an hour gap for magnesium itself; the spacing figures come from the research cited on what you should not take with magnesium.

When should you stop magnesium or see a GP?

Lower the dose, split it or take it with food if stools loosen; stop if they have not settled within a few days at the lower dose. The prescription monograph's own advice is to interrupt treatment and restart at a reduced dose if diarrhoea occurs.¹² See your GP before starting if you have kidney disease or a heart-rhythm condition, and stop and speak to your GP if you notice palpitations, muscle weakness, or unusual drowsiness or confusion.^{2, 12} Call 111 when the surgery is closed, and 999 for difficulty breathing, collapse or sudden confusion. Iron supplements side effects follows the same pattern.

What we don't know

Only one supplier-funded single dose has compared the common salts at a matched elemental dose in healthy adults, so no form can be called gentler or harsher than another. The forum symptoms have not been measured, and the NHS says there is not enough evidence on high doses taken for a long time.⁴

Frequently asked questions

Does magnesium glycinate cause diarrhoea?

Any magnesium salt can at a high enough dose, and the glycinate data are small: 300 mg a day caused no more nausea or diarrhoea than placebo in 80 pregnant women, and a glycine chelate was better tolerated than oxide in 12 people with part of the bowel removed.^{9, 8}

Can magnesium give you a headache?

Rarely recorded: one of 29 women in a hot-flush pilot stopped magnesium oxide because of headache, and no trial we read found headache more often on magnesium than on placebo.¹¹

Does magnesium make you tired?

Fatigue is listed as rare or very rare in the prescription magnesium monograph, and no supplement trial we read recorded tiredness as an adverse event; "Magnesium contributes to a reduction of tiredness and fatigue" is the register's wording for the nutrient.¹²

How do you stop magnesium diarrhoea?

Lower the elemental dose, split it across the day, and take it with food; the European committee found a divided dose better tolerated and the bowel adapting within days.¹ The nutrition panel's "magnesium ... mg" line, not the compound weight, is the figure to read; how to choose a magnesium supplement shows where it is.

Can magnesium cause heart palpitations?

Not as a recorded side effect of a supplement in any trial we read; the heart is affected by very low magnesium and by dangerously high blood magnesium, the latter seen mainly with kidney impairment.² Stop the supplement and see your GP if palpitations start.

Sources

1. Scientific Committee on Food. Tolerable upper intake level of magnesium, 2001 (EFSA compendium, 2006). efsa.europa.eu.
2. Aal-Hamad AH, et al. Hypermagnesemia in clinical practice. Medicina (Kaunas), 2023. PubMed 37512002.
3. Garrison SR, et al. Magnesium for skeletal muscle cramps. Cochrane Database of Systematic Reviews, 2020. PubMed 32956536.
4. NHS. Vitamins and minerals: others (magnesium). Page last reviewed 3 August 2020 (review overdue; re-read 12 October 2026). nhs.uk.
5. Peikert A, et al. Prophylaxis of migraine with oral magnesium: results from a prospective, multi-center, placebo-controlled and double-blind randomized study. Cephalalgia, 1996. PubMed 8792038.
6. Park H, et al. A double-blind placebo-controlled study of magnesium supplements to reduce menopausal hot flashes. Menopause, 2015. PubMed 25423327.
7. Expert Group on Vitamins and Minerals. Safe Upper Levels for Vitamins and Minerals, 2003, magnesium risk assessment. UK Government Web Archive.
8. Schuette SA, et al. Bioavailability of magnesium diglycinate vs magnesium oxide in patients with ileal resection. JPEN, 1994. PubMed 7815675.
9. Supakatisant C, Phupong V. Oral magnesium for relief in pregnancy-induced leg cramps: a randomised controlled trial. Maternal and Child Nutrition, 2015. PubMed 22909270.
10. Pajuelo D, et al. Nutrients, 2024. PubMed 39770988. Supplier-funded single-dose crossover, 40 adults.
11. Park H, et al. A pilot phase II trial of magnesium supplements to reduce menopausal hot flashes in breast cancer patients. Supportive Care in Cancer, 2011. PubMed 21271347.
12. British National Formulary. Magnesium aspartate. bnf.nice.org.uk.
13. British National Formulary. Magnesium hydroxide, bnf.nice.org.uk; magnesium citrate with sodium picosulfate, bnf.nice.org.uk.
14. Morishita D, et al. Senna versus magnesium oxide for the treatment of chronic constipation: a randomised, placebo-controlled trial. American Journal of Gastroenterology, 2021. PubMed 32969946.
15. NHS. Constipation. Page last reviewed 26 October 2023. nhs.uk.
16. Liamis G, et al. An overview of diagnosis and management of drug-induced hypomagnesemia. Pharmacology Research and Perspectives, 2021. PubMed 34278747.
17. NHS. Taking alendronic acid with other medicines and herbal supplements. Page last reviewed 7 May 2025. nhs.uk.

This article is general information, not medical advice.