# Magnesium side effects and how to avoid them

Source: https://magnesiumguide.uk/safety/side-effects/
Author: Nell Hartley · Updated 2026-11-04

**Short answer:** The commonest side effects of magnesium supplements are diarrhoea, loose stools, stomach cramps and nausea, especially with magnesium oxide, citrate or sulphate and at doses over 400 mg. Taking it with food, splitting the dose or switching to glycinate usually helps. Overdose is rare and mainly affects people with kidney disease.

## What are the common side effects?

- diarrhoea or loose stools
- stomach cramps and wind
- nausea

The NHS notes that taking more than 400 mg for a short time can cause diarrhoea. In trials, how often this happens depends on dose and form: in a migraine trial using 600 mg a day of magnesium citrate, 18.6% of people had diarrhoea and 4.7% had stomach irritation. In a trial of 800 to 1,200 mg a day of magnesium oxide, diarrhoea was more common than on placebo.

## Which forms are most likely to cause them?

| Form | Gut effects |
| --- | --- |
| [Oxide](/forms/magnesium-oxide/), hydroxide, sulphate | Strongly laxative at higher doses |
| [Citrate](/forms/magnesium-citrate/), chloride | Can loosen stools |
| Glycinate, malate, taurate, L-threonate | Usually gentle |

## How can you avoid magnesium side effects?

1. **Take it with food.**
2. **Split the dose:** two doses of 150 mg are easier than one of 300 mg.
3. **Lower the dose.** Many people need only a small top-up: see [dosage and timing](/how-to-take/dosage-and-timing/).
4. **Switch form,** to [glycinate](/forms/magnesium-glycinate/) for example.
5. **Count every source.** Some antacids, laxatives and multivitamins also contain magnesium.

## What are the signs of too much magnesium?

High blood magnesium (hypermagnesaemia) is rare but serious. Signs include:

- nausea and vomiting
- flushing
- drowsiness, confusion or unusual weakness
- low blood pressure, slow or irregular heartbeat
- difficulty breathing

It mostly happens in people with kidney disease, or with very large doses of magnesium laxatives or antacids. A 2018 case report described a death from hypermagnesaemia after long-term laxative use in a woman with normal kidney function. If you have these symptoms, stop taking magnesium and get urgent medical help by calling 111, or 999 in an emergency.

## Who is at higher risk of side effects?

People with kidney disease, older adults (kidney function falls with age), people with long-standing constipation using magnesium laxatives every day, and people taking several magnesium-containing products at once. See [is magnesium safe?](/safety/) and [medicines and magnesium](/safety/drug-interactions/).

## Frequently asked questions

### Why does magnesium give me diarrhoea?

The magnesium your gut doesn't absorb holds water in the bowel, which softens stools. It is the same effect that makes magnesium a laxative, and it is stronger with poorly absorbed forms such as oxide and with larger doses.

### Can magnesium make you tired?

It isn't a common side effect at normal doses. Drowsiness and weakness can be signs of too much magnesium in the blood, which is rare and mainly happens with very high doses or kidney problems. If you feel unusually weak or drowsy, stop taking it and seek advice.

### Can magnesium cause heart palpitations?

At normal doses this is unusual. Very high blood magnesium can slow the heart and cause irregular rhythms. Low magnesium can also cause palpitations. If you have palpitations with dizziness, chest pain or fainting, get urgent help.

### Can magnesium cause a headache or dizziness?

It's uncommon at normal doses. Dizziness can be a sign of low blood pressure from too much magnesium, especially with kidney disease, so stop and seek advice if it happens with nausea, flushing or weakness.

## References

1. [NHS. Vitamins and minerals: others (magnesium).](https://www.nhs.uk/conditions/vitamins-and-minerals/others/)
2. [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.](https://pubmed.ncbi.nlm.nih.gov/8792038/)
3. [Park H et al. A double-blind placebo-controlled study of magnesium supplements to reduce menopausal hot flashes (NCCTG N10C2). Menopause, 2015.](https://pubmed.ncbi.nlm.nih.gov/25423327/)
4. [Bokhari SR et al. Fatal hypermagnesemia due to laxative use. American Journal of the Medical Sciences, 2018.](https://pubmed.ncbi.nlm.nih.gov/29661354/)
5. [National Institutes of Health Office of Dietary Supplements. Magnesium: fact sheet for health professionals.](https://ods.od.nih.gov/factsheets/Magnesium-HealthProfessional/)