Is magnesium safe? Who should avoid supplements

What is a safe amount?

The NHS says that 400 mg a day or less of magnesium from supplements is unlikely to cause any harm, and that the long-term effects of higher doses are not known. Food magnesium doesn’t count, because healthy kidneys quickly get rid of any excess from food.

Who should not take magnesium supplements?

Ask your doctor before taking magnesium if you:

• have kidney disease or reduced kidney function. This is the most important caution, because the kidneys are the body’s only way of clearing extra magnesium.
• have a heart rhythm problem or a condition that causes muscle weakness, because high magnesium can affect both.
• take regular medicines, especially antibiotics, osteoporosis tablets, levothyroxine, diuretics or acid-reducing tablets. See medicines it affects.
• are pregnant or breastfeeding.

What are the side effects?

Mostly digestive: diarrhoea, loose stools, stomach cramps and nausea, more common with oxide, citrate, chloride and sulphate, and at doses above 400 mg. See side effects for how to avoid them.

Can you overdose?

Yes, though it is rare. Hypermagnesaemia (too much magnesium in the blood) causes nausea, flushing, drowsiness, muscle weakness, low blood pressure and, in severe cases, breathing problems and heart rhythm changes. It is mostly seen in people with kidney failure, or after large doses of magnesium laxatives or antacids. A 2018 case report described a woman who died after long-term magnesium laxative use despite normal kidney function. Get urgent medical help if you develop these symptoms while taking magnesium.

What about pregnancy?

A 2014 Cochrane review of 10 trials in 9,090 pregnant women found no clear evidence that magnesium supplements improved outcomes for mothers or babies, and no clear harm. Magnesium given by drip in hospital is an established treatment for severe pre-eclampsia, but that is a different situation. If you are pregnant, ask your midwife before taking a supplement.