Who should not take magnesium supplements? Know these 5 risks before you reach for this popular 'cure-all' for better sleep & fewer cramps

Magnesium is an essential mineral involved in hundreds of processes in the body, and supplements are often promoted for benefits ranging from muscle function to sleep and relaxation. But more is not always better. For some people, particularly those with certain medical conditions or those taking specific medicines, magnesium supplements can cause problems or interfere with treatment.

The National Institutes of Health (NIH) notes that healthy kidneys normally remove excess magnesium through urine. However, high doses from supplements or medicines can cause diarrhoea, nausea and abdominal cramping, while extremely high levels can lead to low blood pressure, irregular heartbeat and, in severe cases, cardiac arrest.

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Here are some groups who should be particularly careful before taking magnesium supplements.

People with kidney problems should be cautious with magnesium

The kidneys play an important role in removing excess magnesium from the body. When kidney function is impaired, the body may not be able to clear magnesium efficiently, increasing the risk of magnesium toxicity or hypermagnesaemia.

According to the NIH, the risk of magnesium toxicity increases in people with impaired renal function or kidney failure. Severe excess magnesium can cause symptoms including nausea, vomiting, muscle weakness, difficulty breathing, low blood pressure and irregular heartbeat.

This does not mean that everyone with kidney problems must automatically avoid magnesium. However, supplementation should be discussed with a doctor, particularly when kidney function is significantly reduced.

People taking certain medicines need to check for magnesium interactions

Magnesium supplements can interfere with the absorption or effectiveness of some medicines. The NIH specifically lists certain antibiotics, including tetracyclines and quinolones, among medicines that can interact with magnesium. Magnesium can bind to these drugs in the digestive tract, reducing their absorption.

Diuretics can also affect magnesium levels. Some types increase magnesium loss through urine, while potassium-sparing diuretics can reduce magnesium excretion. This means the effect can vary depending on the medicine being taken.

People taking regular prescription medicines should therefore tell their doctor or pharmacist before starting a magnesium supplement.

People with a slow heart rate should not self-medicate

Magnesium is important for normal muscle and nerve function, including the heart. However, very high magnesium levels can affect cardiovascular function.

The NIH notes that magnesium toxicity can cause hypotension and irregular heartbeat, particularly when intake is excessive.

Anyone with an existing heart rhythm or conduction problem should therefore avoid taking high-dose magnesium supplements without medical advice. Symptoms such as unusual weakness, dizziness or fainting after taking a supplement warrant medical attention.

People with diarrhoea or sensitive digestion may experience more stomach problems

Magnesium supplements are well known for their laxative effects. High supplemental intakes can cause diarrhoea, nausea and abdominal cramping, according to the NIH. Some magnesium compounds are more likely to cause these gastrointestinal effects because unabsorbed magnesium salts draw water into the intestine.

For someone who already experiences frequent loose stools or digestive sensitivity, adding a magnesium supplement could potentially make symptoms worse.

It is worth remembering that diarrhoea after taking magnesium is not necessarily a sign that the supplement is "cleansing" the body. It can simply be a side effect of the supplement.

People with low blood pressure should be careful with high doses

Very high magnesium levels can cause hypotension, or abnormally low blood pressure. The risk becomes more concerning when excessive magnesium intake is combined with other factors or medicines that can also lower blood pressure.

People who frequently experience light-headedness or fainting, or those taking blood-pressure medicines, should speak to a healthcare professional before starting high-dose magnesium supplements.

What about magnesium and depression?

There has been research into magnesium supplementation as a possible addition to depression treatment. In a 2023 randomised, double-blind trial involving 60 people with major depressive disorder who were already taking SSRIs, 30 participants received magnesium and 30 received a placebo for six weeks. The researchers found that Beck Depression Inventory scores were significantly lower in the magnesium group at weeks four and six.

The researchers concluded that magnesium might have potential as an adjunct treatment for people with major depressive disorder taking SSRIs. However, this was a relatively small study and does not mean magnesium can replace antidepressant treatment or that everyone with depression should take it.

Another randomised trial involving people with depression and magnesium deficiency also reported improvements after supplementation, highlighting the importance of considering whether a person is actually deficient.

How much magnesium from supplements is too much?

The NIH lists 350 mg per day for adults as the tolerable upper intake level for magnesium specifically from dietary supplements and medicines. This limit does not include magnesium naturally present in food.

That distinction matters because getting magnesium from foods is generally considered safe for healthy people, while excessive supplemental magnesium can produce side effects.


Disclaimer: This article is for informational purposes only and not a substitute for professional medical advice. Always seek the advice of your doctor with any questions about a medical condition.