Magnesium sulfate is an essential medication used in UK hospitals for serious conditions including eclampsia, severe pre-eclampsia, acute severe asthma, and fetal neuroprotection during preterm birth. Whilst highly effective when properly administered, magnesium sulfate side effects can range from common, manageable symptoms such as flushing and nausea to serious complications including respiratory depression and cardiac disturbances. Understanding these potential adverse effects, recognising warning signs, and knowing when to seek medical help are crucial for patient safety. This article examines the side effects of magnesium sulfate, explains why they occur, identifies who should avoid this medication, and outlines how healthcare professionals manage and minimise risks during treatment.

What Is Magnesium Sulfate and How Does It Work?

Magnesium sulfate is an inorganic salt containing magnesium, sulfur, and oxygen, used in clinical practice for several important indications. In the UK, it is primarily administered intravenously or intramuscularly in hospital settings, though oral preparations exist for specific purposes such as bowel preparation or as a laxative.

The medication works through multiple mechanisms depending on its clinical application. As an anticonvulsant, magnesium sulfate acts as a physiological calcium antagonist and NMDA receptor antagonist, reducing neuronal excitability and preventing seizures. This makes it the first-line treatment for eclampsia and severe pre-eclampsia in pregnant women, as recommended by NICE guideline NG133. In acute severe asthma, it causes bronchodilation by relaxing smooth muscle in the airways, typically used as an adjunctive therapy when standard treatments have not achieved adequate response, as recommended in BTS/SIGN guidelines.

Magnesium sulfate is also used for fetal neuroprotection in women at risk of imminent preterm birth before 30 weeks' gestation, as recommended by NICE guideline NG25. Additionally, it replaces magnesium in cases of documented hypomagnesaemia, which can occur with certain medications, malabsorption disorders, or chronic alcohol use. Some preparations are also licensed for the treatment of torsades de pointes.

The therapeutic effects depend on achieving appropriate serum magnesium concentrations. Normal serum magnesium levels range from 0.7 to 1.0 mmol/L, whilst therapeutic levels for conditions like eclampsia are typically 2.0 to 3.5 mmol/L. Understanding how magnesium sulfate works helps explain why certain side effects occur, particularly when levels become elevated beyond the therapeutic range.

Common Side Effects of Magnesium Sulfate

When administered therapeutically, magnesium sulfate commonly produces several predictable side effects related to its physiological actions. These effects are generally dose-dependent and more pronounced with intravenous administration.

Cardiovascular and circulatory effects are amongst the most frequent. Patients often experience:

• Flushing and warmth, particularly in the face and upper body, caused by peripheral vasodilation

• Hypotension (low blood pressure), which may cause dizziness or light-headedness

• Reduced heart rate, as magnesium affects cardiac conduction

Gastrointestinal symptoms occur commonly, especially with oral preparations:

• Nausea and vomiting, affecting patients receiving intravenous magnesium

• Diarrhoea, particularly with oral magnesium sulfate due to its osmotic laxative effect

• Abdominal discomfort or cramping

Neuromuscular effects reflect magnesium's action at the neuromuscular junction:

• Muscle weakness or heaviness, particularly in the limbs

• Drowsiness and lethargy, which can be pronounced

• Reduced or absent deep tendon reflexes, routinely monitored during treatment

Local reactions at the injection site may include pain, irritation, or a burning sensation during intravenous infusion. Slowing the infusion rate often alleviates this discomfort.

Most common side effects are temporary and resolve once treatment is completed or the infusion rate is adjusted. Healthcare professionals monitor patients closely during magnesium sulfate administration, checking vital signs, reflexes, and respiratory function regularly. Patients receiving treatment should report any uncomfortable symptoms to their medical team, as adjustments to the infusion rate or supportive measures can often improve tolerability whilst maintaining therapeutic benefit.

Serious Side Effects and When to Seek Medical Help

Whilst magnesium sulfate is generally safe when properly monitored, serious adverse effects can occur, particularly with excessive serum concentrations. These complications require immediate medical attention and are why the medication is typically administered in hospital settings with appropriate monitoring.

Hypermagnesaemia (elevated magnesium levels) produces progressively severe effects:

• Respiratory depression is one of the most serious complications, occurring when magnesium levels reach approximately 6 mmol/L. This manifests as slow, shallow breathing or difficulty breathing and constitutes a medical emergency

• Cardiac complications including severe bradycardia (very slow heart rate), heart block, or cardiac arrest can occur at levels above 12.5 mmol/L

• Complete loss of deep tendon reflexes typically occurs around 5 mmol/L, precedes respiratory depression and serves as an important warning sign

• Profound muscle weakness or paralysis affecting respiratory muscles

Pulmonary oedema (fluid in the lungs) has been reported, particularly in pregnant women receiving magnesium sulfate alongside large volumes of intravenous fluids. Symptoms include severe breathlessness, coughing, and chest tightness.

Immediate medical help is essential if you experience:

• Difficulty breathing or shortness of breath

• Chest pain or irregular heartbeat

• Severe dizziness or fainting

• Confusion or altered consciousness

• Inability to move limbs or severe weakness

• Significantly reduced urine output

In hospital settings, healthcare professionals monitor for these complications through regular assessment of vital signs, deep tendon reflexes, respiratory rate, and urine output. Serum magnesium levels are checked periodically. Calcium gluconate (10 mL of 10% solution given intravenously over approximately 10 minutes) is the specific antidote for magnesium toxicity and should be readily available wherever magnesium sulfate is administered. NICE guideline NG133 emphasises the importance of appropriate monitoring protocols to prevent serious adverse events, particularly in obstetric settings where magnesium sulfate is frequently used.

Who Should Avoid Magnesium Sulfate?

Certain individuals should not receive magnesium sulfate, or require special precautions and modified dosing due to increased risk of adverse effects. Healthcare professionals carefully assess each patient's suitability before administration.

Absolute contraindications (situations where magnesium sulfate should not be used) include:

• Myasthenia gravis, an autoimmune neuromuscular disorder, as magnesium sulfate can precipitate severe muscle weakness or myasthenic crisis

• Heart block or severe bradycardia, since magnesium further slows cardiac conduction

• Severe renal impairment, as the kidneys are the primary route of magnesium excretion, significantly increasing toxicity risk

• Known hypersensitivity to magnesium sulfate or any formulation components

Relative contraindications (requiring careful consideration and monitoring) include:

• Moderate renal impairment: Magnesium is primarily excreted by the kidneys, so reduced renal function increases the risk of accumulation and toxicity. Dose adjustments and more frequent monitoring are essential

• Respiratory disease: Pre-existing respiratory conditions increase vulnerability to respiratory depression

Special populations requiring caution:

Elderly patients may be more susceptible to side effects due to age-related decline in renal function and increased sensitivity to neuromuscular effects. Lower doses and enhanced monitoring are often appropriate.

Patients taking certain medications require careful assessment, particularly those receiving:

• Calcium channel blockers (increased risk of hypotension and cardiac effects)

• Neuromuscular blocking agents (enhanced muscle relaxation)

• Aminoglycosides and tetracyclines (increased neuromuscular blockade)

• Cardiac glycosides like digoxin (increased risk of heart block)

• Other central nervous system depressants

Before receiving magnesium sulfate, inform your healthcare team about all medical conditions, particularly kidney problems, heart conditions, or muscle disorders, and provide a complete list of current medications including over-the-counter preparations and supplements. This information enables appropriate risk assessment and monitoring strategies to be implemented.

Managing and Reducing Side Effects

Effective management of magnesium sulfate side effects involves both preventive strategies and prompt intervention when adverse effects occur. In hospital settings, healthcare professionals employ several evidence-based approaches to optimise safety and tolerability.

Monitoring protocols form the cornerstone of safe magnesium sulfate administration:

• Regular vital signs assessment: Blood pressure, heart rate, and respiratory rate checked hourly or more frequently

• Deep tendon reflex testing: Loss of reflexes indicates rising magnesium levels and precedes respiratory depression

• Urine output monitoring: Ensuring adequate renal function and magnesium excretion (minimum 30 mL/hour)

• Serum magnesium level checks: Periodic blood tests to maintain therapeutic range

Dose optimisation strategies help minimise side effects:

• Slower infusion rates reduce the intensity of flushing, nausea, and injection site discomfort

• Dose adjustments based on renal function, body weight, and clinical response

• Using the lowest effective dose for the shortest necessary duration

Symptomatic management addresses common side effects:

• For nausea: Anti-emetic medications can be prescribed

• For hypotension: Intravenous fluids, position changes (lying flat), or dose reduction

• For injection site discomfort: Slowing infusion rate or using larger veins

• For muscle weakness: Reassurance and assistance with mobility

Emergency interventions for serious toxicity include:

• Immediate cessation of magnesium sulfate infusion

• Intravenous calcium gluconate (10 mL of 10% solution over approximately 10 minutes), which rapidly reverses magnesium's effects

• Respiratory support including supplemental oxygen or mechanical ventilation if needed

• Haemodialysis in severe cases, particularly with renal impairment

Patient involvement is important. If receiving magnesium sulfate, report any concerning symptoms immediately to nursing or medical staff. After discharge, contact your GP or NHS 111 if you experience persistent symptoms. Never attempt to self-administer magnesium sulfate outside supervised medical settings, as the risks of serious complications are significant without appropriate monitoring.

Suspected side effects can be reported via the MHRA Yellow Card Scheme at yellowcard.mhra.gov.uk or via the Yellow Card app.

Frequently Asked Questions