Magnesium supplements

Key points about magnesium supplements

  • Magnesium is vital for overall body function.
  • Most people can get enough magnesium from a healthy, balanced diet but there are some times when magnesium supplements may be helpful.
  • Find out how to take it safely and possible side effects.
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Magnesium is an important nutrient your body needs to stay healthy. Magnesium is central to many processes in your body, including regulating muscle and nerve function, and making proteins and bone. Magnesium plays an important role in glucose metabolism, DNA repair and mood.

The recommended dietary intake (RDI) each day for adult men is 420 mg and, for women is 320 mg (elemental magnesium). You can usually get enough magnesium from eating a healthy, balanced diet and a variety of foods, including the following:

  • Legumes, nuts, seeds, whole grains, and green leafy vegetables (such as spinach).
  • Fortified breakfast cereals and other fortified foods – ‘fortified’ means that extra vitamins, minerals or micronutrients are added when the food is made.
  • Milk, yoghurt, and some other milk products.

Getting more magnesium than we need from food is fine, as the body can easily rid itself of what it doesn’t need.

Low magnesium levels in your blood are usually due to your body releasing too much magnesium from the bowel or kidneys, because of an underlying medical condition. It is rare for a healthy person to have low magnesium simply from not taking in enough from their diet. This is because nearly all foods contain some magnesium and healthy kidneys are very good at regulating and maintaining magnesium.

Magnesium deficiency (low levels) can cause many different symptoms – typical signs are tremors, convulsions, muscle spasms, weakness, delirium, seizures (fits), and abnormal heart rhythm. It can cause vague symptoms such as tiredness (fatigue) and drowsiness. If you have migraines and asthma, you may also have low magnesium levels 

Having a mildly low magnesium level is not necessarily bad for you. There is some controversy whether mildly low magnesium can have an anti-inflammatory and antioxidant effect.

You may be more at risk of low magnesium levels if you:
  • are recovering from diarrhoea (runny poo) or vomiting, if you have certain bowel disorders such as coeliac diseaseinflammatory bowel disease (IBD), or if you’ve had gastric bypass surgery
  • drink excessive alcohol
  • have certain rare genetic conditions.

 Description of the evidence
Muscle cramps in pregnancy
Despite common opinion, magnesium supplements don’t appear to help for general night-time (nocturnal) leg cramps. For pregnancy-associated rest cramps, there is conflicting evidence and further research is needed.
Severe asthma
In severe asthma attacks in hospital magnesium given through a vein or inhaled via a nebuliser may be effective. However there are more effective treatments and so it isn’t often used. For patients with asthma who are not in hospital, oral magnesium doesn’t help.
People who get migraines often have lower magnesium levels than others. There is some evidence that taking a magnesium supplement can be effective in some people for both preventing and treating migraine.
Many antacids have magnesium in them. Magnesium has been shown to be helpful in indigestion (dyspepsia).
It is sometimes used for constipation but there is no high quality evidence.
Sleep problems
Despite being commonly used for insomnia, there is no evidence that it actually helps. 
People with diabetes often have lower levels of magnesium. Because of its role in glucose metabolism, increased magnesium intake may be beneficial for people with type 2 diabetes for improving insulin sensitivity and blood glucose control.

Magnesium that is naturally present in food is not harmful and does not need to be limited. If you are healthy, your kidneys can get rid of any extra magnesium in your urine. But magnesium in dietary supplements and medicines should not be consumed in amounts above the upper limit (350 mg per day for adults), unless recommended by your healthcare provider. 

One of the first side effects of taking too much magnesium from supplements is diarrhoea (runny poo), which explains why magnesium is used in some remedies for constipation. Other side effects might include nausea (feeling sick) and stomach cramps (pains), low blood pressure or muscle weakness.

In more severe cases, high magnesium levels can cause confusion, paralysis, coma, and heart rhythm changes.

Check with your doctor or pharmacist before you start taking a magnesium supplement to make sure that it won’t affect any medications or medical conditions you have.

You can buy magnesium supplements without a prescription from pharmacies, health stores or on the internet.

Magnesium supplements come in a variety of different preparations with them being attached to different “salts” such as magnesium aspartate and magnesium citrate. There are also long acting preparations, eg, magnesium chloride and magnesium lactate.

Each of these has a different amount of actual magnesium (called elemental magnesium). The elemental magnesium dose is what is important and the label on the product should state this.

There is no good evidence to support any one preparation over another. Poor absorption can be a problem in all preparations and this may be improved by taking it during meals and not all in one single daily intake. Magnesium oxide seems to be the most poorly absorbed of the different preparations.

Magnesium is also included in some laxatives and some products for treating heartburn and indigestion.

When buying magnesium supplements, always read the label and check with your doctor or pharmacist first. For adults, if you take a supplement, it’s recommended you should have no more than 350 mg (elemental magnesium) a day.


Magnesium fact sheet for consumers(external link)(external link) National Institute of Health, US, 2020
5 questions to ask about your medications(external link)(external link)(external link) Health Quality and Safety Commission, NZ, 2019 English(external link), te reo Māori(external link)


  1. Magnesium(external link) National Institutes of Health
  2. A guide to magnesium(external link) Healthy Food Guide, NZ
  3. Effect of magnesium oxide supplementation on nocturnal leg cramps – A randomized clinical trial(external link) JAMA Internal Medicine, 2016
  4. Cochrane Magnesium for skeletal muscle cramps(external link) Cochrane Database of Systematic Reviews 2020
  5. Intravenous and nebulised magnesium sulphate for acute asthma: systematic review and meta-analysis(external link) Emerg Med J. 2007 
  6. Oral magnesium and vitamin C supplements in asthma – a parallel group randomized placebo-controlled trial (external link)Clin Exp Allergy. 2003
  7. Role of magnesium in the pathogenesis and treatment of migraine(external link) Expert Rev Neurother. 2009 
  8. On-demand treatment of acute heartburn with the antacid hydrotalcite compared with famotidine and placebo: randomized double-blind cross-over study. J Clin Gastroenterol. 2007
  9. Efficacy and safety of traditional medical therapies for chronic constipation – systematic review(external link) Am J Gastroenterol. 2005 
  10. Oral magnesium supplementation for insomnia in older adults – a systematic review & meta-analysis(external link) BMC Complement Med Ther. 2021 
  11. Effect of magnesium supplementation on glucose metabolism in people with or at risk of diabetes – a systematic review and meta-analysis of double-blind randomized controlled trials(external link) Eur J Clin Nutr. 2016 

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