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 consists of one magnesium ion and two chloride ions, and is naturally found in seawater and underground brine deposits. In the UK, magnesium chloride is available primarily as an oral supplement for preventing and treating magnesium deficiency (hypomagnesaemia), which can result from inadequate dietary intake, malabsorption disorders, certain medications, or chronic conditions. Understanding its uses, forms, and safety considerations helps patients and healthcare professionals make informed decisions about magnesium 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 one 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 0.70–1.00 mmol/L in most UK laboratories, with hypomagnesaemia defined as levels below 0.70 mmol/L.

Magnesium chloride is one of several magnesium salts available for supplementation, alongside magnesium sulphate, citrate, oxide, and glycerophosphate, each with different properties and uses. When dissolved in water, magnesium chloride dissociates into magnesium and chloride ions, which can then be absorbed by the body.

In the UK, magnesium products are available in two regulatory categories: prescription-only medicines licensed by the Medicines and Healthcare products Regulatory Agency (MHRA), and food supplements regulated under the UK Food Supplements Regulations. Most over-the-counter magnesium products, including many magnesium chloride preparations, fall into the latter category. For clinical use in hospitals, particularly for intravenous administration, magnesium sulphate rather than magnesium chloride is the standard preparation used in UK practice.

Medical Uses and Indications

Magnesium chloride is primarily indicated for the prevention and treatment of hypomagnesaemia (low magnesium levels), which may arise from inadequate dietary intake, malabsorption disorders, chronic diarrhoea, or increased renal losses. Conditions such as Crohn's disease, coeliac disease, chronic alcoholism, and poorly controlled diabetes mellitus are commonly associated with magnesium depletion. Certain medications can also precipitate magnesium deficiency, including proton pump inhibitors, loop diuretics, aminoglycosides, amphotericin B, cisplatin, cyclosporine, tacrolimus, and some chemotherapy agents.

In acute care settings, intravenous magnesium may be administered to correct severe hypomagnesaemia, particularly when accompanied by symptoms such as muscle cramps, tremors, cardiac arrhythmias, or seizures. In the UK, magnesium sulphate (not magnesium chloride) is the standard intravenous preparation used in hospital settings. NICE guidance (NG133) recommends magnesium sulphate for the management of eclampsia and severe pre-eclampsia in obstetric patients, where it acts as an anticonvulsant. Magnesium sulphate is also used in the treatment of torsades de pointes, a potentially life-threatening cardiac arrhythmia, as recommended in Resuscitation Council UK Advanced Life Support guidelines.

Beyond deficiency states, magnesium has applications in supportive care. It may be used as an adjunct in the management of chronic constipation, though magnesium hydroxide is more commonly prescribed for this indication. In critical care, intravenous magnesium sulphate infusions are sometimes employed in the management of severe asthma exacerbations that have not responded adequately to initial treatment, as recommended by the British Thoracic Society and NICE guidance.

There is some evidence suggesting a role for magnesium supplementation in migraine prophylaxis, though NICE guidance does not routinely recommend it as a first-line preventive treatment.

Patients experiencing symptoms of severe hypomagnesaemia such as seizures, syncope, sustained arrhythmias, or severe muscle weakness should seek urgent medical assessment.

Forms and Routes of Administration

Magnesium is available in several pharmaceutical formulations, each suited to different clinical scenarios and patient needs. The choice of formulation depends on the severity of deficiency, the urgency of treatment, and patient-specific factors such as gastrointestinal function and venous access.

Oral formulations are the most common route for long-term supplementation and mild to moderate deficiency. These include:

• Tablets and capsules: Available in various magnesium salts, with elemental magnesium content varying widely between products. Dosing is typically expressed in millimoles (mmol) of magnesium, with usual supplementation doses ranging from 5–20 mmol daily for deficiency. Patients should check the elemental magnesium content on product labels

• Oral solutions: Liquid preparations that may be better tolerated in patients with swallowing difficulties or those requiring flexible dosing

• Effervescent preparations: Dissolve in water to create a palatable drink, often preferred by patients who dislike swallowing tablets

Oral magnesium absorption varies between different salts and individuals, and can be reduced in patients with gastrointestinal disorders. Taking magnesium with food can improve tolerability but may slightly reduce absorption.

Intravenous formulations are reserved for severe deficiency, acute symptomatic hypomagnesaemia, or situations where oral administration is not feasible. In UK practice, intravenous magnesium sulphate (not magnesium chloride) is the standard preparation used in hospitals. It is typically administered as a diluted infusion over 30 minutes to several hours, depending on the dose and clinical urgency. Rapid intravenous administration can cause flushing, hypotension, and cardiac complications, so careful monitoring of ECG and respiratory status is essential.

Topical preparations, including magnesium oils, gels, and bath salts, are marketed for transdermal absorption. However, there is limited robust clinical evidence supporting significant systemic absorption or therapeutic efficacy via this route. Such products are regulated as cosmetics or general products in the UK, not as medicines, and should not be relied upon to treat magnesium deficiency.

Safety Considerations and Side Effects

Magnesium supplements are generally well tolerated when used appropriately, but both oral and intravenous formulations carry potential adverse effects that patients and healthcare professionals should be aware of. Understanding these risks enables safer prescribing and helps patients recognise when to seek medical advice.

Common side effects of oral magnesium include:

• Gastrointestinal disturbances: Diarrhoea, nausea, abdominal cramping, and bloating are the most frequently reported adverse effects. These are dose-dependent and often resolve with dose reduction or taking the supplement with food

• Metallic taste: Some patients report an unpleasant taste, particularly with liquid formulations

These effects are generally mild and self-limiting, but persistent or severe gastrointestinal symptoms warrant medical review, as they may indicate intolerance or excessive dosing.

Serious adverse effects are uncommon but can occur, particularly with intravenous administration or in patients with impaired renal function. Hypermagnesaemia (elevated serum magnesium) can develop when magnesium intake exceeds the body's capacity for excretion. Symptoms include:

• Muscle weakness and reduced reflexes

• Nausea and vomiting

• Hypotension and bradycardia

• Respiratory depression

• Cardiac arrest (in severe cases)

Patients with chronic kidney disease are at significantly increased risk of hypermagnesaemia and should only use magnesium supplements under close medical supervision with regular monitoring of serum magnesium and renal function.

Contraindications and cautions include:

• Severe renal impairment (eGFR <30 mL/min/1.73m²)

• Myasthenia gravis (magnesium can worsen muscle weakness)

• Heart block or severe bradycardia

Drug interactions are important to consider. Magnesium can reduce the absorption of certain medications, including:

• Levothyroxine (separate by at least 4 hours)

• Bisphosphonates (take separately as directed)

• Tetracycline antibiotics (take at least 2 hours before or 4-6 hours after magnesium)

• Quinolone antibiotics (separate by 2-4 hours)

• Iron and zinc supplements (take at different times)

Parenteral magnesium may potentiate the effects of neuromuscular blocking agents during anaesthesia and can enhance the hypotensive effects of calcium channel blockers.

Patient safety advice: Individuals should contact their GP if they experience persistent diarrhoea, muscle weakness, or irregular heartbeat while taking magnesium supplements. Call 999 for severe chest pain, severe breathlessness, collapse or seizures, and seek urgent care for sustained palpitations or severe weakness. Pregnant and breastfeeding women should consult a healthcare professional before starting supplementation. Patients should not exceed recommended doses without medical supervision, and those with kidney problems must seek specialist advice before using any magnesium supplement.

Suspected adverse reactions to magnesium products should be reported via the MHRA Yellow Card scheme.

Frequently Asked Questions