Low magnesium, or hypomagnesaemia, occurs when serum magnesium levels drop below the normal range of 0.7–1.0 mmol/L. This essential mineral supports over 300 enzymatic reactions, including muscle and nerve function, heart rhythm regulation, and bone health. Whilst true deficiency is uncommon in the UK, certain groups face higher risk due to inadequate dietary intake, increased losses, or impaired absorption. Understanding what causes a low magnesium level is vital, as prolonged deficiency can lead to serious complications such as cardiac arrhythmias, seizures, and metabolic disturbances. Early recognition and appropriate management help prevent these outcomes and support overall wellbeing.

What Is Low Magnesium and Why Does It Matter?

Low magnesium, medically termed hypomagnesaemia, occurs when serum magnesium levels fall below the normal range (typically 0.7-1.0 mmol/L, though reference ranges may vary between laboratories). Magnesium is the fourth most abundant mineral in the human body and plays a crucial role in over 300 enzymatic reactions. It is essential for maintaining normal muscle and nerve function, supporting a healthy immune system, keeping the heart rhythm steady, and contributing to bone strength. Additionally, magnesium helps regulate blood glucose levels and is involved in energy production and protein synthesis.

While true magnesium deficiency is uncommon in the general UK population, certain groups are at higher risk. Routine blood tests do not always include magnesium levels, and serum magnesium may not accurately reflect total body stores, as most magnesium resides within cells and bone tissue rather than in the bloodstream. This means that a person can have normal blood magnesium levels whilst still experiencing intracellular deficiency. Alternative measurements of magnesium status are not routinely recommended in primary care.

The consequences of prolonged magnesium deficiency can be significant. Low magnesium levels have been associated with cardiovascular disease, type 2 diabetes, osteoporosis, and migraine headaches, although causality has not been definitively established for these conditions. In severe cases, hypomagnesaemia can lead to life-threatening complications including cardiac arrhythmias and seizures. Understanding what causes low magnesium is therefore essential for both prevention and appropriate management. Early identification and correction of magnesium deficiency can help prevent these complications and improve overall health outcomes.

Common Causes of Low Magnesium Levels

Magnesium deficiency typically arises from three main mechanisms: inadequate dietary intake, increased losses from the body, or impaired absorption from the gastrointestinal tract. Understanding these pathways helps identify individuals at risk and guides appropriate intervention.

Dietary insufficiency is a common contributor, particularly in Western diets that are high in processed foods and low in magnesium-rich whole foods. Good dietary sources of magnesium include:

• Green leafy vegetables (spinach, kale)

• Nuts and seeds (almonds, pumpkin seeds)

• Whole grains (brown rice, wholemeal bread)

• Legumes (black beans, chickpeas)

• Dark chocolate and avocados

Certain lifestyle factors can also deplete magnesium stores. Excessive alcohol consumption is a significant cause, as alcohol increases urinary magnesium excretion and impairs intestinal absorption. Chronic alcohol use can lead to both acute and chronic magnesium deficiency. Similarly, poorly controlled diabetes results in increased urinary losses due to osmotic diuresis—when excess glucose in the urine draws magnesium along with it.

Excessive sweating during intense physical activity or in hot climates can contribute to magnesium depletion, though this is rarely the sole cause unless combined with inadequate intake. Athletes and individuals who engage in prolonged endurance exercise may be at higher risk, particularly if dietary intake is inadequate. Age-related factors also play a role, as older adults often have reduced dietary intake and decreased intestinal absorption of magnesium.

Malnutrition and refeeding syndrome are important causes of magnesium deficiency. During refeeding of severely malnourished individuals, rapid shifts in metabolism can lead to dangerous drops in magnesium and other electrolytes, requiring careful monitoring and replacement.

Medical Conditions That Lead to Magnesium Deficiency

Several medical conditions can significantly impair magnesium absorption or increase its loss from the body, leading to chronic deficiency. Gastrointestinal disorders are among the most common culprits. Conditions affecting the small intestine—where magnesium is primarily absorbed—can substantially reduce magnesium uptake. These include:

• Coeliac disease: Damage to the intestinal villi impairs nutrient absorption

• Crohn's disease: Inflammation and surgical resection of the small bowel reduce absorptive capacity

• Chronic diarrhoea: Regardless of cause, persistent diarrhoea leads to excessive magnesium losses

• Short bowel syndrome: Following extensive intestinal resection, absorption is compromised

Renal (kidney) disorders that affect tubular function can cause magnesium wasting. Inherited renal tubular disorders, such as Gitelman syndrome and Bartter syndrome, cause excessive renal magnesium losses. It's important to note that while these conditions increase magnesium excretion, advanced chronic kidney disease (CKD) typically reduces magnesium excretion and can actually lead to elevated magnesium levels, particularly in patients with severely reduced kidney function.

Endocrine conditions also affect magnesium homeostasis. Poorly controlled diabetes mellitus causes osmotic diuresis leading to magnesium depletion. Hyperparathyroidism and hyperthyroidism can both increase urinary magnesium excretion. Additionally, hungry bone syndrome—which can occur after parathyroid surgery—may cause temporary hypomagnesaemia as bone rapidly takes up minerals during remineralisation.

Other relevant conditions include acute pancreatitis, where magnesium can be sequestered in areas of fat necrosis, and malabsorption syndromes of any aetiology. Refeeding syndrome in severely malnourished patients can cause dangerous drops in magnesium levels. Patients with these conditions should be monitored for magnesium deficiency, and supplementation may be required as part of their ongoing management plan.

Medications That Can Lower Magnesium

Numerous commonly prescribed medications can contribute to magnesium deficiency, either by increasing urinary losses or by impairing intestinal absorption. Healthcare professionals should be aware of these interactions, particularly in patients taking multiple medications or those with other risk factors for hypomagnesaemia.

Proton pump inhibitors (PPIs), such as omeprazole, lansoprazole, and esomeprazole, are among the most widely prescribed medications in the UK. The MHRA has highlighted that severe hypomagnesaemia has been reported in patients taking PPIs for at least 3 months, but most cases occur after a year of treatment. The MHRA advises that healthcare professionals should consider measuring magnesium levels before starting long-term PPI therapy and periodically during treatment, especially in patients taking digoxin or medications that may cause hypomagnesaemia, such as diuretics.

Diuretics are another important class. Loop diuretics (furosemide, bumetanide) and thiazide diuretics (bendroflumethiazide, indapamide) increase renal magnesium excretion. Patients on long-term diuretic therapy, particularly for heart failure or hypertension, may require magnesium monitoring and supplementation.

Antibiotics can also affect magnesium levels. Aminoglycosides (gentamicin, tobramycin) cause renal magnesium wasting through tubular damage.

Other medications associated with low magnesium include:

• Chemotherapy agents: Cisplatin is particularly notable for causing magnesium wasting

• EGFR inhibitors: Cetuximab and panitumumab require regular magnesium monitoring during treatment

• Immunosuppressants: Ciclosporin and tacrolimus increase renal losses

• Antifungals: Amphotericin B causes renal magnesium wasting

Patients taking these medications should be counselled about maintaining adequate dietary magnesium intake, and clinicians should maintain a low threshold for checking magnesium levels if symptoms develop. Patients and healthcare professionals are encouraged to report suspected adverse drug reactions to the MHRA through the Yellow Card Scheme (yellowcard.mhra.gov.uk).

Recognising the Symptoms of Low Magnesium

The clinical presentation of magnesium deficiency varies considerably depending on the severity and duration of depletion. Mild deficiency may be entirely asymptomatic or produce only subtle, non-specific symptoms that are easily attributed to other causes. As deficiency progresses, symptoms become more pronounced and potentially serious.

Early or mild symptoms may include:

• Fatigue and general weakness

• Loss of appetite and nausea

• Muscle cramps, particularly in the legs

• Tingling or numbness in the extremities

• Personality changes, including apathy or irritability

Moderate to severe deficiency can produce more concerning manifestations. Neuromuscular symptoms become prominent, including muscle twitching (fasciculations), tremor, and increased muscle tone. Patients may experience tetany—involuntary muscle contractions—which can be particularly noticeable in the hands and feet (carpopedal spasm). Chvostek's sign (facial twitching when the facial nerve is tapped) and Trousseau's sign (carpopedal spasm induced by inflating a blood pressure cuff) may be elicited on examination.

Cardiac manifestations are among the most serious complications. Magnesium deficiency can cause various arrhythmias, including atrial fibrillation, ventricular tachycardia, and torsades de pointes—a potentially fatal rhythm disturbance. ECG changes may include prolonged QT interval. Patients may experience palpitations, chest discomfort, or dizziness. Importantly, hypomagnesaemia often coexists with other electrolyte abnormalities, particularly hypokalaemia (low potassium) and hypocalcaemia (low calcium), which can be difficult to correct until magnesium levels are normalised.

Severe, prolonged deficiency may lead to seizures, confusion, and altered mental status. Some observational studies suggest associations between chronic magnesium deficiency and conditions such as migraine, depression, and anxiety, though causality remains uncertain and requires further investigation.

When to See Your GP About Low Magnesium

Knowing when to seek medical advice is crucial for timely diagnosis and management of magnesium deficiency. You should contact your GP if you experience persistent symptoms suggestive of low magnesium, particularly if you have risk factors such as gastrointestinal disease, diabetes, or are taking medications known to affect magnesium levels.

Specific situations warranting GP consultation include:

• Persistent muscle cramps, twitching, or spasms that are not relieved by simple measures

• Unexplained fatigue or weakness affecting daily activities

• Numbness or tingling in the hands, feet, or around the mouth

• Irregular heartbeat or palpitations

• Persistent nausea or loss of appetite

• Known risk factors (chronic diarrhoea, long-term PPI use, diuretic therapy)

Seek urgent medical attention (call 999 or attend A&E) if you experience:

• Severe muscle spasms or seizures

• Chest pain or severe palpitations

• Difficulty breathing

• Confusion or altered consciousness

Your GP can arrange appropriate investigations, typically including blood tests to measure serum magnesium, as well as other electrolytes (potassium, calcium, phosphate), renal function, and possibly parathyroid hormone (PTH) and vitamin D levels. An ECG may be performed if you have palpitations or suspected arrhythmias. In some cases, urinary magnesium measurement may help distinguish between renal and gastrointestinal causes of deficiency.

Management depends on the severity of deficiency and underlying cause. Mild deficiency may be addressed through dietary modification and oral magnesium supplements. Moderate to severe deficiency, particularly if symptomatic or associated with cardiac arrhythmias, may require intravenous magnesium replacement, usually administered in hospital according to BNF guidance and Resuscitation Council UK protocols.

Long-term management involves addressing the underlying cause where possible—for example, optimising control of diabetes, reviewing medication regimens, or treating gastrointestinal conditions. Regular monitoring may be necessary for patients with ongoing risk factors. Your GP may refer you to a specialist if you have persistent or severe deficiency, recurrent arrhythmias, unclear cause, or suspected renal tubular disorder.

If taking magnesium supplements, separate them by at least 2-4 hours from medications such as levothyroxine, tetracycline antibiotics, quinolone antibiotics, and bisphosphonates, as magnesium can reduce their absorption. Self-prescribing high-dose magnesium supplements without medical supervision is not recommended, as excessive intake can cause diarrhoea and, in rare cases, serious complications, particularly in people with kidney disease.

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