Magnesium deficiency: symptoms, causes, tests and treatment

The BNSSG guidance says most patients with hypomagnesaemia have no symptoms, and that symptoms usually do not appear until the serum magnesium is below 0.4 mmol/L. How a person feels may depend more on how fast the deficiency developed or on the total body deficit than on the blood level alone.

When symptoms occur the guidance lists anorexia, nausea, confusion, weakness, ataxia (unsteadiness), paraesthesia (pins and needles), tetany (muscle spasm), tremor and muscle fasciculations (twitching). Cardiac arrhythmias may occur, and with very low levels seizures, drowsiness and coma. The Specialist Pharmacy Service classes a level of 0.5 to 0.7 mmol/L as mild, with no symptoms or non-specific ones such as lethargy, muscle cramps or weakness, and below 0.5 mmol/L as severe, with neurological symptoms such as nystagmus, tetany, seizures and arrhythmias.

Because these symptoms are non-specific, they cannot on their own tell you that your magnesium is low.

According to the Specialist Pharmacy Service, hypomagnesaemia can occur through decreased intake, intestinal losses such as diarrhoea, medication side effects and renal (kidney) losses. The BNSSG guidance lists malnutrition or diarrhoea, certain medicines, certain inherited kidney conditions such as Gitelman's syndrome, and having had part of the small intestine removed or bypassed.

The medicines named are proton pump inhibitors, diuretics, antibiotics (aminoglycosides), some chemotherapy drugs and immunosuppressants such as tacrolimus. If you take one of these and develop symptoms, it is worth telling your GP rather than stopping the medicine yourself.

The BNSSG guidance says management should be based on treating the underlying cause and supplementation according to magnesium level and the presence of symptoms. Oral supplements are usually sufficient for levels between 0.4 and 0.7 mmol/L, whereas a level below 0.4 mmol/L, or symptoms, may require admission for initial treatment through a drip.

Oral magnesium salts cause diarrhoea, so should be given with food if possible. Normalisation of magnesium may take 6 to 8 weeks, with weekly monitoring, and long-term maintenance treatment may be required if a reversible cause is not identified. The Specialist Pharmacy Service adds that untreated hypomagnesaemia can lead to life-threatening arrhythmias, which is why it is treated and monitored by clinicians rather than self-managed.

Magnesium is a mineral that, according to the NHS, helps turn the food we eat into energy and makes sure the parathyroid glands, which produce hormones important for bone health, work normally. The BNSSG ICB guidance for GPs describes it as an essential constituent of many enzyme systems, particularly those involved in energy generation, with the largest stores in the skeleton.

Magnesium deficiency in a clinical sense is hypomagnesaemia: a low level of magnesium in the blood. It is usually defined as a level below 0.7 mmol/L. It may also cause secondary low calcium and low potassium, which is why those are checked at the same time.

It is worth separating this from the everyday use of the phrase. Feeling tired, having cramps or sleeping badly are common and have many causes; a magnesium level is only low when a blood test shows it.

It is diagnosed by a blood test. In most cases, the BNSSG guidance says, the cause can be obtained from the history, and if no cause is apparent further tests may be required. Because low magnesium can cause low potassium and low calcium, urea and electrolytes and calcium are checked. A 24-hour urine collection for magnesium may be used if the cause is not clear, to distinguish kidney losses from other causes.

The Specialist Pharmacy Service notes that the reference range for serum magnesium and dosing recommendations may vary locally.

The NHS lists spinach, nuts and wholemeal bread among the good sources of magnesium, and gives the amount needed as 300mg a day for men and 270mg a day for women aged 19 to 64. It says you should be able to get all you need from a varied, balanced diet.

Taking high doses (more than 400mg) for a short time can cause diarrhoea, and there is not enough evidence to say what the effects of long-term high doses are. The Department of Health and Social Care advice quoted by the NHS is that taking 400mg or less a day of magnesium from supplements is unlikely to cause harm. If you have kidney disease or take regular medicines, check with a pharmacist or GP before starting a magnesium supplement.

Many everyday problems, such as tiredness, muscle cramps or poor sleep, have other causes, and low magnesium can only be confirmed by a blood test. If you have persistent muscle cramps, twitching, tremor, weakness or tingling, particularly if you take a medicine linked to low magnesium (see the causes above) or have long-term diarrhoea, speak to your GP.

The BNSSG ICB guidance for GPs advises considering emergency admission for a level below 0.4 mmol/L or if the person is symptomatic, and the Specialist Pharmacy Service lists seizures and heart rhythm problems as features of severe low magnesium. If someone has a seizure, collapses, or has chest pain with an irregular or very slow heartbeat, call 999.

Do not stop a prescribed medicine without advice. If you think a medicine may be lowering your magnesium, ask your GP or pharmacist to review it.

On the NHS a GP orders a magnesium level when there is a clinical reason, such as a relevant medicine, symptoms or abnormal potassium or calcium. Private blood testing services can measure magnesium on request, but a single result needs interpreting alongside symptoms, medicines and kidney function, and a low result still needs a cause to be sought. Red blood cell or hair magnesium tests are not described in the guidance used here.