What causes a low magnesium level is a question many people ask when faced with unexplained fatigue, muscle cramps, or other non-specific symptoms. Low magnesium, or hypomagnesaemia, occurs when serum magnesium falls below 0.7 mmol/L and can result from inadequate dietary intake, increased losses through the kidneys or gastrointestinal tract, or impaired absorption. Common culprits include certain medications (such as proton pump inhibitors and diuretics), chronic medical conditions (including diabetes and inflammatory bowel disease), and lifestyle factors like excessive alcohol consumption. Understanding the underlying causes is essential for effective prevention and treatment, as severe deficiency can lead to serious complications including cardiac arrhythmias and seizures.

What Is Low Magnesium and Why Does It Matter?

Low magnesium, medically termed hypomagnesaemia, occurs when serum magnesium levels fall below 0.7 mmol/L (with normal range typically 0.7-1.0 mmol/L). 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.

Magnesium deficiency is more common in certain populations, particularly hospitalised patients, older adults, and those with specific medical conditions, though its prevalence in the general population is less certain. Routine blood tests do not always include magnesium levels. Furthermore, serum magnesium may not accurately reflect total body stores, as most magnesium resides within cells and bone tissue rather than in the bloodstream. Serum magnesium remains the standard test in UK clinical practice.

Observational studies have found associations between low magnesium levels and increased risk of cardiovascular disease, type 2 diabetes, osteoporosis, and migraine headaches, though these relationships do not necessarily prove causation. In severe cases, hypomagnesaemia can lead to potentially serious complications including cardiac arrhythmias and seizures. Understanding what causes low magnesium is therefore important 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 can contribute, particularly in 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.

Increased renal losses represent another important pathway. The kidneys normally reabsorb approximately 95% of filtered magnesium, but this process can be disrupted by various factors. Poorly controlled diabetes mellitus causes osmotic diuresis, leading to excessive magnesium loss in urine. High sodium intake and hypercalcaemia can increase urinary magnesium losses. Age-related changes in kidney function may also reduce the body's ability to conserve magnesium, making older adults particularly vulnerable to deficiency due to reduced intake, absorption, comorbidities, and medication use.

Other important causes include chronic laxative use and refeeding syndrome, where rapid reintroduction of nutrition in malnourished individuals can cause significant shifts in electrolytes including magnesium.

Medical Conditions That Lead to Magnesium Deficiency

Several medical conditions can significantly impair magnesium absorption or increase its loss from the body. Gastrointestinal disorders are among the most common culprits. Chronic diarrhoea from any cause results in substantial magnesium loss through the stool. Conditions such as Crohn's disease, ulcerative colitis, and coeliac disease can reduce magnesium absorption, particularly when the small intestine is affected. Inflammatory bowel disease may cause losses through diarrhoea and malabsorption, especially with small intestinal involvement or resections. Similarly, patients who have undergone bowel resection surgery may have insufficient intestinal length to absorb adequate magnesium.

Malabsorption syndromes more broadly can lead to deficiency. These include conditions affecting the small intestine where magnesium is primarily absorbed. Chronic pancreatitis with pancreatic insufficiency can impair magnesium absorption, as can small intestinal bacterial overgrowth (SIBO). Fat malabsorption conditions are particularly problematic because magnesium can bind to unabsorbed fatty acids, forming insoluble soaps that are excreted rather than absorbed.

Endocrine disorders also play a significant role. Poorly controlled type 2 diabetes mellitus causes glycosuria (glucose in urine), which creates an osmotic diuresis that carries magnesium out with it. Primary hyperparathyroidism and hypercalcaemia can increase urinary magnesium losses through effects on renal tubular handling.

Genetic disorders affecting renal magnesium handling, whilst rare, can cause severe deficiency. Gitelman syndrome and Bartter syndrome are inherited tubular disorders that result in excessive urinary magnesium wasting. These conditions typically present in childhood or early adulthood with persistent hypomagnesaemia despite supplementation. Familial hypomagnesaemia with hypercalciuria and nephrocalcinosis (FHHNC) is another rare genetic cause that requires specialist management.

Other clinical contexts where hypomagnesaemia is common include refeeding syndrome and hungry bone syndrome (following parathyroidectomy).

Medications That Can Lower Magnesium

Numerous commonly prescribed medications can contribute to magnesium deficiency through various mechanisms. Proton pump inhibitors (PPIs), such as omeprazole, lansoprazole, and esomeprazole, are widely used for acid reflux and peptic ulcer disease. Long-term PPI use (typically beyond one year) has been associated with hypomagnesaemia, likely due to impaired intestinal magnesium absorption. The MHRA has issued guidance recommending that magnesium levels should be considered before starting PPI treatment in patients taking digoxin or other medications that may cause hypomagnesaemia, and periodically during prolonged treatment.

Diuretics are another important medication class affecting magnesium balance. Loop diuretics (furosemide, bumetanide) and thiazide diuretics (bendroflumethiazide, indapamide) increase urinary magnesium excretion by interfering with renal tubular reabsorption. Patients on long-term diuretic therapy for heart failure or hypertension should have their electrolytes, including magnesium, monitored regularly. Potassium-sparing diuretics such as amiloride can help conserve magnesium.

Chemotherapy agents, particularly platinum-based drugs such as cisplatin, can cause significant renal magnesium wasting that may persist long after treatment completion. Epidermal growth factor receptor (EGFR) inhibitors like cetuximab also commonly cause hypomagnesaemia. Immunosuppressants including calcineurin inhibitors (ciclosporin, tacrolimus) used in transplant patients can impair renal magnesium reabsorption.

Other medications that may lower magnesium include:

• Aminoglycoside antibiotics (gentamicin)

• Amphotericin B (antifungal)

• Pentamidine (antiprotozoal)

• Foscarnet (antiviral)

Patients taking these medications should be monitored appropriately, and supplementation considered when indicated. If you suspect you're experiencing side effects from any medication, you can report them through the MHRA Yellow Card Scheme.

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 cause only subtle, non-specific symptoms that are easily overlooked. These can include:

• Fatigue and general weakness

• Loss of appetite

• Nausea

• Mild muscle cramps or twitching

• Difficulty sleeping or restlessness

As magnesium levels decline further, moderate deficiency produces more recognisable neuromuscular symptoms. Patients may experience muscle cramps, particularly in the legs and feet, which can be severe and occur frequently, especially at night. Muscle twitching (fasciculations), tremor, and generalised weakness become more pronounced. Some individuals report numbness or tingling sensations (paraesthesia) in the extremities.

Severe hypomagnesaemia can lead to serious complications requiring urgent medical attention. Neurological manifestations may include confusion, personality changes, seizures, and altered consciousness. Cardiovascular effects are particularly concerning and include cardiac arrhythmias such as atrial fibrillation, ventricular tachycardia, and torsades de pointes—a potentially fatal rhythm disturbance. If you experience palpitations, syncope (fainting), or suspected arrhythmia, an ECG may be needed. Severe deficiency also commonly causes secondary hypocalcaemia (low calcium) and hypokalaemia (low potassium), which are often resistant to correction until magnesium is repleted.

Evidence linking non-specific symptoms (such as anxiety, depression, or general fatigue) to measured hypomagnesaemia is inconsistent. These symptoms are non-specific and testing is needed for confirmation. Symptoms alone cannot reliably diagnose magnesium deficiency, and blood testing is essential. The presence of risk factors (medications, medical conditions) alongside suggestive symptoms should prompt investigation.

When to See Your GP About Low Magnesium

You should arrange to see your GP if you experience persistent symptoms suggestive of magnesium deficiency, particularly if you have risk factors such as chronic gastrointestinal conditions, diabetes, or are taking medications known to affect magnesium levels. Specific symptoms warranting medical review include frequent or severe muscle cramps, persistent muscle twitching, unexplained fatigue, or numbness and tingling in your extremities.

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

• Seizures or fits

• Severe confusion or altered consciousness

• Irregular or rapid heartbeat

• Chest pain

• Severe muscle weakness affecting breathing or swallowing

Your GP will take a detailed medical history, including your diet, alcohol consumption, medications, and any underlying health conditions. They will perform a physical examination and arrange appropriate blood tests. Investigation of suspected electrolyte disturbances typically includes serum magnesium alongside other electrolytes (sodium, potassium, calcium), renal function tests, and potentially parathyroid hormone levels if calcium is also abnormal. An ECG may be recommended if you have cardiac symptoms.

If hypomagnesaemia is confirmed, your GP will investigate the underlying cause. This may involve:

• Reviewing and potentially adjusting medications

• Assessing for malabsorption (coeliac serology, faecal elastase)

• Checking diabetes control (HbA1c)

• Measuring urinary magnesium excretion if renal wasting is suspected

Treatment depends on severity. Mild deficiency may be managed with dietary modification and oral magnesium supplements (typically magnesium oxide, citrate, or glycerophosphate). Use caution with magnesium supplements if you have chronic kidney disease and discuss with your clinician; diarrhoea is a common side effect of oral magnesium. Moderate to severe deficiency, particularly with symptoms, may require intravenous magnesium replacement, usually administered in hospital. Your GP will arrange appropriate follow-up to monitor response to treatment and prevent recurrence. Patients with chronic conditions causing ongoing magnesium loss may require long-term supplementation and regular monitoring.

Frequently Asked Questions