Magnesium chloride (MgCl₂) is an inorganic salt compound used as a source of supplemental magnesium, an essential mineral involved in over 300 enzymatic reactions in the body. It plays a critical role in energy production, muscle and nerve function, blood glucose control, and bone health. In the UK, magnesium chloride is available as both licensed medicinal products and food supplements, though for therapeutic purposes, the British National Formulary (BNF) primarily recommends other magnesium salts such as magnesium sulfate for injectable use and magnesium aspartate for oral replacement therapy.

What Is Magnesium Chloride?

Magnesium chloride is an inorganic salt compound with the chemical formula MgCl₂, consisting of one magnesium ion and two chloride ions. It is a naturally occurring mineral found in seawater and underground brine deposits, and serves as a source of supplemental magnesium. As an essential electrolyte, magnesium plays a vital role in numerous physiological processes throughout the human body.

In the body, magnesium acts as a cofactor for more than 300 enzymatic reactions, including those involved in energy production, protein synthesis, muscle and nerve function, blood glucose control, and blood pressure regulation. It is also essential for the structural development of bone and is required for the synthesis of DNA, RNA, and the antioxidant glutathione. Normal serum magnesium levels typically range between 0.75–1.05 mmol/L, and deficiency can lead to significant clinical consequences.

Magnesium chloride is generally more soluble than some other magnesium forms such as magnesium oxide, which may influence its absorption. When dissolved in water, it dissociates into magnesium and chloride ions. In the UK, magnesium chloride is available both as licensed medicinal products (regulated by the Medicines and Healthcare products Regulatory Agency, MHRA) and as food supplements (regulated under food law). For therapeutic use, the British National Formulary (BNF) primarily lists magnesium sulfate for injectable preparations and magnesium aspartate or glycerophosphate for oral replacement therapy, rather than magnesium chloride specifically.

Medical Uses and Indications

Magnesium chloride's primary clinical application is the treatment and prevention of hypomagnesaemia (low magnesium levels), which may result from inadequate dietary intake, gastrointestinal disorders affecting absorption (such as Crohn's disease or coeliac disease), chronic diarrhoea, or increased renal losses due to certain medications including diuretics and proton pump inhibitors.

It's important to note that for several acute medical emergencies in UK practice, magnesium sulfate (not magnesium chloride) is the standard treatment. These include:

• Severe acute asthma that has not responded adequately to initial bronchodilator therapy. NICE guidelines recommend considering intravenous magnesium sulfate for patients with acute severe or life-threatening asthma.

• Cardiac arrhythmias, particularly torsades de pointes, a potentially life-threatening ventricular tachycardia associated with QT interval prolongation. Resuscitation Council UK guidelines recommend magnesium sulfate in this context.

• Eclampsia and pre-eclampsia in pregnant women, conditions characterised by dangerously elevated blood pressure and the risk of seizures. NICE guidance specifically recommends magnesium sulfate for these indications.

When treating hypomagnesaemia, healthcare professionals should check calcium and potassium levels concurrently, as deficiencies often coexist. Intravenous replacement is typically considered when serum magnesium is below 0.5 mmol/L or in symptomatic patients. In patients with confirmed magnesium deficiency who have alcohol dependence, replacement therapy may be indicated, but routine supplementation without documented deficiency is not standard UK practice.

Healthcare professionals should assess magnesium levels through blood tests before initiating replacement therapy, particularly in patients with renal impairment, and address the underlying causes of deficiency.

Forms and Routes of Administration

Magnesium is available in multiple pharmaceutical formulations, with selection depending on the severity of deficiency, the urgency of treatment, and the patient's ability to tolerate oral medication.

Oral formulations are the most common for routine supplementation and mild to moderate hypomagnesaemia. These include tablets, capsules, and oral solutions. A typical magnesium chloride tablet might contain 535 mg of magnesium chloride hexahydrate, which provides approximately 64 mg (2.6 mmol) of elemental magnesium. In UK practice, licensed oral magnesium products (such as magnesium aspartate) typically express dosing in mmol rather than mg. Dosing should follow the specific product's Summary of Product Characteristics (SmPC) or BNF guidance. Oral magnesium should ideally be taken with food to enhance absorption and minimise gastrointestinal side effects.

Intravenous (IV) magnesium is reserved for severe deficiency, acute medical emergencies, or when oral administration is not feasible. In UK clinical practice, magnesium sulfate is the standard intravenous preparation, not magnesium chloride. IV formulations allow for rapid correction of magnesium levels and are administered as either slow bolus injections or continuous infusions, depending on clinical urgency. In hospital settings, magnesium levels are monitored closely during IV administration, particularly in patients with renal impairment, as the kidneys are the primary route of magnesium excretion.

Topical preparations, including magnesium oils, gels, and bath flakes, are marketed for transdermal absorption, though evidence supporting their efficacy remains limited. These products are not recommended for treating documented magnesium deficiency and should not be considered a substitute for proper medical treatment of hypomagnesaemia. Patients considering topical magnesium should discuss this with their GP or pharmacist, particularly if they have underlying medical conditions requiring formal magnesium replacement.

Safety Considerations and Side Effects

Magnesium chloride is generally well tolerated when used appropriately, but both oral and intravenous formulations can cause adverse effects, particularly when taken in excessive amounts or by patients with certain medical conditions. Understanding these safety considerations is essential for both healthcare professionals and patients.

Common side effects of oral magnesium chloride include gastrointestinal disturbances such as diarrhoea, nausea, abdominal cramping, and bloating. These effects are dose-dependent and often resolve with dose reduction or by taking the medication with food. Diarrhoea is particularly common because magnesium has an osmotic effect in the intestinal lumen, drawing water into the bowel. Patients should be advised to start with lower doses and gradually increase as tolerated.

Hypermagnesaemia (elevated magnesium levels) is the most serious potential complication, particularly with intravenous administration or in patients with impaired renal function. Early symptoms include nausea, flushing, and a feeling of warmth. As levels rise, patients may experience muscle weakness, hypotension (low blood pressure), bradycardia (slow heart rate), drowsiness, and reduced reflexes. Severe hypermagnesaemia can lead to respiratory depression, complete heart block, cardiac arrest, and coma. For this reason, intravenous magnesium must only be administered in monitored healthcare settings with appropriate resuscitation facilities available.

Contraindications and cautions include severe renal impairment (eGFR <30 mL/min/1.73m²), myasthenia gravis, heart block, and active gastrointestinal obstruction. Magnesium can interact with several medications, including bisphosphonates, tetracyclines, quinolones, calcium channel blockers, levothyroxine, and iron supplements. Oral magnesium should be separated from these medications by at least 2-4 hours to avoid reduced absorption. When given intravenously, magnesium requires caution with neuromuscular blocking agents and CNS depressants.

When to seek medical advice: Patients should contact their GP if they experience persistent diarrhoea, signs of dehydration, muscle weakness, irregular heartbeat, or difficulty breathing whilst taking magnesium. Call 999 or attend A&E for severe breathlessness, chest pain, collapse/syncope, seizures, or suspected overdose. Those with kidney disease should only use magnesium supplements under medical supervision with regular monitoring of serum magnesium and renal function. Pregnant or breastfeeding women should consult their healthcare provider before using magnesium supplements.

Suspected side effects can be reported via the MHRA Yellow Card Scheme.

Frequently Asked Questions