Magnesium sulfate is an essential medication used in UK hospitals for serious conditions including eclampsia, severe asthma, and fetal neuroprotection during preterm birth. Whilst generally safe under medical supervision, magnesium sulfate side effects can range from mild flushing and nausea to serious complications such as respiratory depression and cardiac problems. Understanding these potential adverse effects, recognising warning signs, and knowing when to seek help are crucial for patient safety. This article examines common and serious side effects, identifies who should avoid this medication, and explains how healthcare teams monitor and manage complications 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 at the neuromuscular junction, 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 when standard treatments have not achieved adequate response, as recommended in the BTS/SIGN British Guideline on Asthma Management.

Magnesium sulfate is also used for fetal neuroprotection when preterm birth is imminent (before 30 weeks), as outlined in NICE guideline NG25. Additionally, it replaces magnesium in cases of documented hypomagnesaemia, which can occur with certain medications, malabsorption disorders, or chronic alcohol use. It may also be used to treat torsades de pointes, a specific type of cardiac arrhythmia.

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. However, clinical monitoring of reflexes, respiratory rate and urine output is more important than targeting specific serum levels. 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

Most patients receiving magnesium sulfate experience some degree of side effects, though many are mild and transient. The frequency and severity often relate to the dose administered, rate of infusion, and individual patient factors.

Cardiovascular effects are among the most frequently observed. Flushing and a sensation of warmth throughout the body occur very commonly, caused by peripheral vasodilation. Patients may also experience hypotension (low blood pressure), particularly if the infusion is administered too rapidly. This can manifest as dizziness or light-headedness. Healthcare professionals monitor blood pressure regularly during administration to detect and manage this promptly.

Gastrointestinal symptoms include nausea, which affects a significant proportion of patients, and occasionally vomiting. These effects result from magnesium's action on the gastrointestinal tract and central nervous system. When taken orally, magnesium sulfate commonly causes diarrhoea due to its osmotic laxative properties, drawing water into the intestinal lumen.

Neuromuscular effects manifest as muscle weakness, reduced deep tendon reflexes, and lethargy. Patients often describe feeling drowsy or experiencing generalised weakness. Sweating is also reported frequently, related to the vasodilatory effects and autonomic nervous system changes.

Local reactions at the injection site may include pain, irritation, or a burning sensation during intravenous administration. Intramuscular injections can be particularly uncomfortable and may cause localised soreness lasting several hours.

These common side effects typically resolve once the infusion is completed or the dose is reduced. However, they require monitoring to ensure they do not progress to more serious complications. Healthcare teams assess patients regularly, checking vital signs and reflexes to maintain safe therapeutic levels whilst minimising adverse effects.

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 may necessitate discontinuation of treatment.

Hypermagnesaemia (elevated magnesium levels) represents the most significant risk. As serum magnesium rises above therapeutic levels, progressive toxicity develops:

• At 3.5–5.0 mmol/L: Loss of deep tendon reflexes, pronounced muscle weakness, and significant drowsiness

• At 5.0–7.5 mmol/L: Respiratory depression, severe hypotension, and electrocardiogram (ECG) changes including prolonged PR interval and widened QRS complex

• Above 7.5 mmol/L: Respiratory arrest, complete heart block, and cardiac arrest

Respiratory complications are particularly concerning. Magnesium sulfate can cause respiratory depression by affecting the respiratory centre in the brainstem and weakening respiratory muscles. Patients may develop slow, shallow breathing or, in severe cases, respiratory failure requiring mechanical ventilation. This risk increases in patients with pre-existing respiratory conditions or when magnesium is combined with other central nervous system depressants.

Cardiac effects beyond hypotension include bradycardia (slow heart rate), arrhythmias, and in extreme cases, cardiac arrest. ECG monitoring may be required during administration, particularly in patients with renal impairment who cannot efficiently excrete magnesium, or those with pre-existing cardiac conditions.

Call 999 or go to A&E immediately if you experience:

• Severe muscle weakness or inability to move

• Difficulty breathing or shortness of breath

• Chest pain or irregular heartbeat

• Confusion or loss of consciousness

• Severe dizziness or fainting

In hospital settings, healthcare professionals monitor for these complications through regular assessment of vital signs, deep tendon reflexes (typically patellar reflex), respiratory rate, and when indicated, serum magnesium levels. Calcium gluconate is kept readily available as an antidote, as calcium directly antagonises magnesium's effects at the neuromuscular junction and can rapidly reverse life-threatening toxicity. The standard dose is 10 mL of 10% calcium gluconate given intravenously over approximately 10 minutes.

Who Should Avoid Magnesium Sulfate?

Certain patient groups require particular caution or should avoid magnesium sulfate entirely due to increased risk of adverse effects or reduced ability to eliminate the medication safely.

Renal impairment represents the most significant contraindication. The kidneys excrete magnesium, and patients with reduced renal function (estimated glomerular filtration rate below 30 mL/min/1.73m²) cannot efficiently clear magnesium from the body. This dramatically increases the risk of hypermagnesaemia and toxicity. If magnesium sulfate is deemed essential in patients with renal impairment, doses must be substantially reduced and serum levels monitored very closely.

Myasthenia gravis and other neuromuscular disorders pose significant risks. Magnesium sulfate can precipitate or worsen muscle weakness in these conditions, potentially leading to myasthenic crisis or respiratory failure. The medication's effects at the neuromuscular junction directly interfere with already compromised neuromuscular transmission.

Heart block or severe cardiac disease requires careful consideration. Magnesium sulfate can worsen conduction abnormalities and should be avoided in patients with second- or third-degree heart block unless a pacemaker is in place. Patients with severe bradycardia or recent myocardial infarction require specialist assessment before administration.

Additional cautions include:

• Concurrent use of calcium channel blockers: Additive effects on cardiac conduction and blood pressure

• Respiratory depression from other causes: Magnesium may compound existing respiratory compromise

• Hypocalcaemia: Low calcium levels may be worsened by magnesium administration

• Concurrent use of neuromuscular blocking agents or aminoglycosides: May potentiate neuromuscular blockade

In pregnancy, whilst magnesium sulfate is used therapeutically for eclampsia prevention and fetal neuroprotection, administration near delivery requires careful timing. Neonates born to mothers receiving magnesium sulfate may exhibit hypotonia, respiratory depression, and reduced reflexes, requiring observation in neonatal units. Healthcare professionals must inform patients about these considerations and conduct thorough medical assessments before initiating treatment.

Managing and Reducing Side Effects

Effective management of magnesium sulfate side effects relies on careful monitoring, appropriate dosing, and prompt intervention when problems arise. Both healthcare professionals and patients play important roles in minimising adverse effects.

Clinical monitoring protocols form the foundation of safe administration. Healthcare teams should:

• Check deep tendon reflexes (usually patellar) before each dose and regularly during infusion—absent reflexes indicate approaching toxicity

• Monitor respiratory rate regularly, maintaining rates above 12 breaths per minute

• Measure urine output, ensuring adequate renal function (typically >30 mL/hour)

• Assess serum magnesium levels when indicated, particularly in prolonged infusions or patients with renal impairment

• Record vital signs including blood pressure and heart rate at frequent intervals

Dose adjustments represent the primary method of reducing side effects. Slowing the infusion rate often alleviates symptoms like flushing, nausea, and hypotension without compromising therapeutic efficacy. In cases of mild toxicity, temporarily stopping the infusion allows magnesium levels to decline naturally through renal excretion.

Symptomatic management addresses specific side effects:

• For hypotension: Intravenous fluids and positioning the patient supine with legs elevated (or in left lateral tilt if pregnant)

• For nausea: Antiemetic medications may be prescribed, though these must be chosen carefully to avoid additional sedation

• For local injection site discomfort: Slowing the infusion rate and ensuring proper dilution

Emergency management of severe toxicity requires immediate action. Calcium gluconate (10 mL of 10% solution administered intravenously over approximately 10 minutes) serves as the specific antidote, rapidly reversing magnesium's effects. This may be repeated if needed according to local protocols. Supportive care may include oxygen therapy, mechanical ventilation for respiratory depression, and cardiac monitoring with treatment of arrhythmias.

Patient education is essential. Individuals receiving magnesium sulfate should understand the importance of reporting symptoms promptly, particularly increasing weakness, breathing difficulties, or severe drowsiness. Following discharge, patients should contact their GP or NHS 111 if they experience persistent symptoms or have concerns about their recovery. Patients can report suspected side effects via the MHRA Yellow Card scheme (online or via the app). Maintaining adequate hydration supports renal magnesium excretion, though this must be balanced against any fluid restrictions related to the underlying condition being treated.

Frequently Asked Questions