Symptoms of magnesium deficiency can range from subtle fatigue and muscle cramps to serious cardiac arrhythmias and seizures. Magnesium deficiency, or hypomagnesaemia, occurs when serum magnesium falls below 0.7 mmol/L. This essential mineral acts as a cofactor in over 300 enzymatic reactions, supporting muscle function, nerve transmission, and cardiovascular health. Whilst true deficiency is uncommon in healthy individuals, certain medications (particularly proton pump inhibitors and diuretics), gastrointestinal disorders, type 2 diabetes, and alcohol use disorder significantly increase risk. Early recognition of symptoms enables prompt treatment and prevents potentially serious complications affecting the heart, muscles, and nervous system.

What Is Magnesium Deficiency?

Magnesium deficiency, clinically termed hypomagnesaemia, occurs when serum magnesium levels fall below the normal reference range of 0.7–1.0 mmol/L. Magnesium is the fourth most abundant cation in the body and the second most abundant intracellular cation after potassium. It serves as a cofactor for over 300 enzymatic reactions, playing crucial roles in protein synthesis, muscle and nerve function, blood glucose control, and blood pressure regulation.

True magnesium deficiency is relatively uncommon in healthy individuals consuming a balanced diet, as the kidneys efficiently regulate magnesium excretion. The body maintains tight homeostatic control of magnesium through intestinal absorption and renal excretion, with approximately 99% of total body magnesium stored in bone, muscle, and soft tissues, leaving only 1% in extracellular fluid.

Diagnosis can be challenging because serum magnesium levels do not always accurately reflect total body magnesium stores. A person may have depleted intracellular magnesium whilst maintaining normal serum levels initially. For this reason, clinical assessment must consider both biochemical measurements and presenting symptoms. When investigating suspected deficiency, additional tests may include measurement of urinary magnesium or fractional excretion to differentiate between renal and gastrointestinal losses. Associated electrolytes (potassium, calcium, phosphate) and relevant hormones (vitamin D, parathyroid hormone) should also be checked.

When magnesium levels become significantly depleted (typically <0.5 mmol/L), multiple organ systems can be affected, leading to a constellation of neuromuscular, cardiovascular, and metabolic disturbances that require prompt recognition and appropriate management.

Common Symptoms of Magnesium Deficiency

The clinical presentation of magnesium deficiency varies considerably depending on the severity and duration of depletion. Early or mild deficiency may produce subtle, non-specific symptoms that are easily overlooked or attributed to other causes. These include:

• Fatigue and weakness – a general sense of tiredness and reduced energy levels

• Loss of appetite – decreased interest in food, which may further compromise nutritional intake

• Nausea – mild gastrointestinal discomfort without specific cause

As deficiency progresses to moderate or severe levels, more distinctive neuromuscular symptoms emerge. Muscle cramps, particularly affecting the legs and feet, are among the most commonly reported complaints. Patients may experience muscle twitching (fasciculations), tremor, or generalised muscle weakness. These symptoms reflect magnesium's essential role in neuromuscular transmission and muscle contraction.

Neurological manifestations can include paraesthesia (tingling or numbness), typically in the extremities, and in severe cases, seizures may occur. Tetany and carpopedal spasm may develop, particularly when hypocalcaemia is also present. Personality changes, apathy, or increased irritability have also been documented, though these are less specific.

Cardiovascular symptoms represent potentially serious complications of magnesium deficiency. Cardiac arrhythmias, including atrial fibrillation, ventricular tachycardia, and characteristic ECG changes (prolonged QT interval), may develop. The prolonged QT interval increases risk of torsades de pointes, particularly in patients taking other QT-prolonging medications. Patients might experience palpitations or irregular heartbeat sensations.

It is important to note that magnesium deficiency rarely occurs in isolation. It frequently coexists with other electrolyte disturbances, particularly hypokalaemia (low potassium) and hypocalcaemia (low calcium), which can be refractory to treatment until magnesium levels are corrected. This interrelationship can complicate the clinical picture and necessitates comprehensive electrolyte assessment when magnesium deficiency is suspected.

When to seek emergency care: Call 999 or go to A&E immediately if experiencing seizures, collapse/syncope, chest pain, or severe palpitations. Seek urgent same-day assessment for suspected significant arrhythmia.

Who Is at Risk of Low Magnesium Levels?

Several population groups and clinical conditions predispose individuals to magnesium deficiency. Understanding these risk factors enables targeted screening and preventive strategies.

Gastrointestinal disorders significantly impair magnesium absorption. Patients with chronic diarrhoea, Crohn's disease, coeliac disease, or those who have undergone bowel resection surgery face increased risk due to reduced intestinal absorption. Acute gastroenteritis with prolonged vomiting and diarrhoea can also precipitate deficiency.

Medication use represents a major iatrogenic cause. Proton pump inhibitors (PPIs), widely prescribed for acid reflux and peptic ulcer disease, can reduce magnesium absorption when used long-term. The MHRA Drug Safety Update has highlighted that hypomagnesaemia can develop after three months of treatment but more commonly occurs after a year of use, and recommends considering magnesium testing before and periodically during prolonged therapy. Loop and thiazide diuretics increase renal magnesium losses and are commonly implicated in deficiency, particularly in elderly patients with heart failure or hypertension. Other medications including certain antibiotics (aminoglycosides, amphotericin B), chemotherapy agents (cisplatin), and immunosuppressants (ciclosporin) can also deplete magnesium.

Type 2 diabetes mellitus is associated with increased urinary magnesium excretion, particularly in poorly controlled diabetes. Alcohol use disorder causes magnesium depletion through multiple mechanisms: poor dietary intake, gastrointestinal losses, and increased renal excretion. Elderly individuals are at heightened risk due to reduced dietary intake, decreased intestinal absorption, increased medication use, and age-related changes in renal function.

Pregnant and lactating women have altered magnesium requirements. Individuals following restrictive diets or with eating disorders may have insufficient magnesium intake. Certain genetic disorders affecting renal magnesium handling (such as Gitelman syndrome) can cause deficiency requiring specialist management.

Chronic kidney disease requires special consideration. While early-stage CKD may sometimes be associated with magnesium losses, advanced CKD more commonly leads to magnesium retention and potential hypermagnesaemia. Patients with kidney disease should always consult their healthcare provider before taking magnesium supplements.

Treatment Options for Magnesium Deficiency

Management of magnesium deficiency depends on severity, underlying cause, and the presence of symptoms. The primary goals are to restore normal magnesium levels, address the underlying cause, and prevent recurrence.

Dietary modification represents the first-line approach for mild deficiency or prevention in at-risk individuals. Magnesium-rich foods include:

• Green leafy vegetables (spinach, kale)

• Nuts and seeds (almonds, cashews, pumpkin seeds)

• Whole grains (brown rice, wholemeal bread)

• Legumes (black beans, chickpeas)

• Fish (mackerel, salmon)

• Dark chocolate and avocados

The Reference Nutrient Intake (RNI) for magnesium in the UK is 300 mg daily for men and 270 mg daily for women.

Oral magnesium supplementation is appropriate for symptomatic mild-to-moderate deficiency. Various formulations are available, including magnesium oxide, citrate, and glycinate. Typical supplementation doses range from 200–400 mg of elemental magnesium daily, though this should be individualised. Gastrointestinal side effects, particularly diarrhoea, are common and dose-dependent; dividing doses or taking with food may improve tolerance.

Important medication interactions: Magnesium supplements can reduce the absorption of certain medications. Take magnesium at least 2 hours apart from tetracycline antibiotics, quinolone antibiotics, bisphosphonates, and levothyroxine.

Intravenous magnesium replacement is reserved for severe deficiency, particularly when accompanied by serious symptoms such as seizures or cardiac arrhythmias, or when oral absorption is compromised. This requires hospital admission for monitoring, as rapid administration can cause adverse effects including hypotension and respiratory depression.

Addressing underlying causes is essential for long-term management. This may involve reviewing and potentially adjusting medications (e.g., switching from PPIs to H2-receptor antagonists where clinically appropriate), optimising management of gastrointestinal disorders, or improving diabetic control. Concurrent electrolyte abnormalities, particularly potassium and calcium, must be corrected simultaneously.

When to seek medical advice: Patients experiencing persistent muscle cramps, significant fatigue, palpitations, or neurological symptoms should consult their GP. Those on long-term PPIs or diuretics should discuss magnesium monitoring with their healthcare provider. The MHRA recommends that magnesium levels should be checked before and during treatment with PPIs in patients expected to be on prolonged therapy, particularly those taking digoxin or other medicines that may cause hypomagnesaemia.

Cautions: Patients with kidney disease should not take magnesium supplements without medical supervision due to the risk of hypermagnesaemia. If you experience side effects from any medicines or supplements, report them through the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk).

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