Magnesium chloride (MgCl₂) is an inorganic salt compound used to treat and prevent magnesium deficiency, a condition that can affect muscle function, heart rhythm, and overall metabolic health. As the fourth most abundant cation in the body, magnesium is essential for over 300 enzymatic reactions, including energy production, nerve transmission, and bone development. In the UK, magnesium chloride is primarily available as a food supplement, whilst other magnesium salts are more commonly used in licensed medicinal products. Understanding its role, appropriate use, and safety considerations is important for both healthcare professionals and patients considering supplementation.

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 an important source of magnesium for treating and preventing magnesium deficiency. In clinical practice, magnesium chloride is commonly available as a dietary supplement, with good absorption properties.

Magnesium itself is the fourth most abundant cation in the body and the second most abundant intracellular cation after potassium. It plays essential roles in over 300 enzymatic reactions, including energy metabolism, protein synthesis, muscle and nerve function, blood glucose control, and blood pressure regulation. Magnesium is also critical for bone structural development and is required for the synthesis of DNA, RNA, and the antioxidant glutathione.

Magnesium chloride contains approximately 12% elemental magnesium by weight, which means that 1 gram of magnesium chloride hexahydrate provides roughly 120 mg of elemental magnesium. This form is absorbed in the gastrointestinal tract, making it an option for oral supplementation. Normal serum magnesium levels typically range between 0.7-1.0 mmol/L.

In the UK, magnesium chloride is primarily available as a food supplement. For licensed medicinal products, other magnesium salts such as magnesium sulfate, magnesium glycerophosphate and magnesium aspartate are more commonly used in clinical practice. Healthcare professionals may recommend magnesium supplements for specific conditions, and over-the-counter preparations are widely available for general supplementation purposes.

Medical Uses and Indications

The primary clinical indication for magnesium supplementation is the treatment and prevention of hypomagnesaemia (low magnesium levels, typically <0.7 mmol/L). Magnesium deficiency can arise from various causes including inadequate dietary intake, malabsorption disorders (such as Crohn's disease or coeliac disease), chronic diarrhoea, alcohol use disorder, uncontrolled diabetes, and certain medications including proton pump inhibitors, thiazide and loop diuretics, aminoglycosides, amphotericin, and cisplatin. Symptoms of deficiency may include muscle cramps, fatigue, weakness, cardiac arrhythmias, and in severe cases, seizures or tetany.

Magnesium supplementation is used in several specific clinical scenarios:

• Cardiovascular conditions: Magnesium plays a role in maintaining normal heart rhythm. In the UK, intravenous magnesium sulfate (not chloride) is used for treating torsades de pointes and other serious arrhythmias according to Resuscitation Council UK guidelines.

• Pre-eclampsia and eclampsia: In the UK, magnesium sulfate is the standard treatment for seizure prophylaxis in severe pre-eclampsia, as recommended by NICE guideline NG133.

• Chronic conditions: Patients with type 2 diabetes, osteoporosis, or migraine may benefit from magnesium supplementation, though evidence quality varies and NICE does not universally recommend routine supplementation for these conditions.

• Medication-induced depletion: Long-term use of certain medications, particularly proton pump inhibitors and diuretics, can lead to magnesium depletion requiring supplementation. The MHRA has issued a Drug Safety Update regarding PPI-associated hypomagnesaemia.

The decision to use magnesium supplements should be based on confirmed or suspected deficiency, typically identified through serum magnesium measurement, though serum levels may not always reflect total body magnesium stores. When hypomagnesaemia is identified, calcium and potassium levels should also be checked, as deficiencies often occur together.

Seek immediate medical attention (call 999) for seizures, syncope, or serious arrhythmias that may be related to severe electrolyte disturbances. Urgent assessment is recommended for marked electrolyte abnormalities or symptomatic deficiency.

Forms and Routes of Administration

Magnesium chloride is available in multiple formulations, each suited to different situations and patient needs. The choice of formulation depends on the severity of deficiency, the patient's ability to take oral medication, and the urgency of treatment.

Oral preparations are the most commonly used forms for mild to moderate deficiency or prophylaxis:

• Tablets and capsules: These provide convenient, measured doses typically ranging from 200–535 mg of magnesium chloride (equivalent to approximately 24–64 mg elemental magnesium). They are suitable for long-term supplementation and prevention.

• Oral solutions: Liquid formulations may be easier to swallow for patients with dysphagia or those who prefer not to take tablets. They allow for flexible dosing adjustments.

• Effervescent preparations: These dissolve in water and may enhance absorption whilst being gentler on the gastrointestinal tract.

Intravenous preparations for severe hypomagnesaemia or emergency situations in UK clinical practice typically use magnesium sulfate, not magnesium chloride. Intravenous magnesium must be administered according to hospital protocols that specify infusion rates and monitoring requirements. Always follow local guidelines for parenteral administration.

Topical formulations, including magnesium chloride solutions or gels applied to the skin, are marketed for muscle relaxation and localised relief. These are typically classified as cosmetic or food supplements in the UK, not licensed medicines. There is limited robust clinical evidence supporting transdermal absorption of therapeutically significant amounts of magnesium for treating deficiency.

Dosing recommendations vary based on the indication and severity of deficiency. For oral supplementation, typical adult doses range from 200–400 mg of elemental magnesium daily, often divided into two or three doses to improve tolerability and absorption. The NHS advises that supplemental intake of up to 400 mg/day of elemental magnesium is unlikely to cause harm for most adults. Higher therapeutic doses should be clinician-directed. Patients should be advised to take oral magnesium with food to reduce gastrointestinal side effects.

Safety Considerations and Side Effects

Magnesium chloride is generally well tolerated when used appropriately, but both healthcare professionals and patients should be aware of potential adverse effects and contraindications. The most common side effects are gastrointestinal in nature, including diarrhoea, nausea, abdominal cramping, and bloating. These effects are dose-dependent and often resolve with dose reduction or by taking the supplement with meals. Diarrhoea is particularly common at higher doses due to the osmotic effect of unabsorbed magnesium in the intestinal lumen.

Contraindications and cautions include:

• Severe renal impairment: Patients with significantly reduced kidney function (eGFR <30 mL/min/1.73m²) are at risk of hypermagnesaemia (elevated magnesium levels) as the kidneys are the primary route of magnesium excretion. Magnesium supplementation should be avoided or used only under specialist supervision in these patients.

• Myasthenia gravis: Magnesium can potentiate neuromuscular blockade and may worsen muscle weakness.

• Heart block: High-dose magnesium can affect cardiac conduction.

Hypermagnesaemia is rare with oral supplementation in patients with normal renal function but can occur with excessive dosing or intravenous administration. Symptoms include nausea, vomiting, facial flushing, hypotension, muscle weakness, and in severe cases, respiratory depression, cardiac arrhythmias, or cardiac arrest. Serum magnesium levels above 1.1 mmol/L warrant clinical attention.

Drug interactions should be considered. Magnesium can reduce the absorption of certain medications including:

• Bisphosphonates: Follow specific product instructions (often requiring separation of 2+ hours)

• Tetracycline antibiotics: Separate by 2-3 hours

• Quinolone antibiotics: Separate by up to 4 hours

• Levothyroxine: Separate by at least 4 hours

• Oral iron: Separate by 2-3 hours

Always follow the specific separation advice in each medication's Patient Information Leaflet or SmPC.

Patient safety advice includes:

• Do not exceed recommended doses without medical supervision

• Inform your GP or pharmacist of all medications and supplements you are taking

• Contact your GP if you experience persistent diarrhoea, severe abdominal pain, or unusual muscle weakness

• Patients with kidney disease should consult their doctor before taking magnesium supplements

• Seek immediate medical attention if you develop symptoms of severe hypermagnesaemia such as difficulty breathing, severe weakness, or irregular heartbeat

Pregnant and breastfeeding women should consult their healthcare provider before taking magnesium supplements, though magnesium is generally considered safe during pregnancy at recommended doses. Regular monitoring of serum magnesium levels may be appropriate for patients on long-term supplementation, particularly those with risk factors for electrolyte disturbances.

Patients are encouraged to report any suspected side effects to the MHRA Yellow Card Scheme (yellowcard.mhra.gov.uk).

Frequently Asked Questions