Magnesium sulfate side effects range from common, manageable reactions to rare but serious complications requiring urgent medical attention. This essential medication is widely used in UK hospitals for pre-eclampsia, severe asthma, and other critical conditions. Whilst most patients experience mild symptoms such as flushing, nausea, or muscle weakness, understanding the full spectrum of potential adverse effects—and recognising warning signs of toxicity—is vital for safe treatment. This article explores how magnesium sulfate works, what side effects to expect, when to seek help, and how healthcare teams monitor and manage risks effectively.

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 most commonly administered intravenously or intramuscularly in hospital settings, though oral preparations exist for specific purposes such as a laxative.

Primary clinical uses include:

• Pre-eclampsia and eclampsia prevention and treatment – the most critical obstetric indication

• Fetal neuroprotection in threatened preterm birth – recommended by NICE, though often used off-label

• Severe acute asthma – as an adjunct bronchodilator when standard treatments prove insufficient (often off-label but guideline-supported)

• Torsades de pointes – a life-threatening cardiac arrhythmia

• Hypomagnesaemia – replacement therapy for magnesium deficiency

• Constipation – oral formulations act as osmotic laxatives

The mechanism of action varies by indication. In pre-eclampsia, magnesium sulfate acts as a central nervous system depressant and anticonvulsant, though the precise mechanism remains incompletely understood. It is thought to reduce neuronal excitability by blocking N-methyl-D-aspartate (NMDA) receptors and modulating calcium channels. In asthma, it promotes bronchodilation by inhibiting calcium influx into smooth muscle cells, leading to relaxation of bronchial airways. In torsades de pointes, it suppresses early after-depolarisations and triggered activity. As a laxative, oral magnesium sulfate draws water into the intestinal lumen through osmotic action, softening stools and stimulating bowel movements.

In the UK, specific magnesium sulfate products are licensed for particular indications, while some common uses (such as in severe asthma and fetal neuroprotection) may be off-label though supported by NICE guidelines and clinical evidence. Understanding how magnesium sulfate works helps contextualise why certain side effects occur and which patients may be at higher risk of adverse reactions.

Common Side Effects of Magnesium Sulfate

When administered therapeutically, magnesium sulfate commonly produces side effects related to its systemic effects on neuromuscular and cardiovascular function. The frequency and severity often depend on the route of administration, dose, and infusion rate.

Frequently reported side effects include:

• Flushing and warmth – particularly at the injection site and spreading throughout the body, caused by peripheral vasodilation

• Nausea and vomiting – a common side effect with intravenous therapy

• Muscle weakness – due to magnesium's effect on neuromuscular transmission

• Drowsiness and lethargy – reflecting central nervous system depression

• Sweating and feeling generally unwell

• Headache – reported by some patients during or after infusion

• Diarrhoea – especially with oral preparations, which is the intended effect when used as a laxative

• Pain at injection site – particularly with intramuscular administration

With intravenous administration, patients often describe a sensation of warmth or heat spreading through the body, which whilst disconcerting is generally harmless. This vasodilatory effect may also cause mild hypotension, typically asymptomatic but occasionally causing dizziness, particularly when standing.

The intensity of side effects usually correlates with serum magnesium levels. Normal serum magnesium is approximately 0.7–1.0 mmol/L. For eclampsia prevention and treatment, the target therapeutic range is typically 2.0–3.5 mmol/L, while for hypomagnesaemia correction, the aim is to restore levels to the normal range. For a single dose in severe asthma, routine serum monitoring is not typically required.

Most common adverse effects are temporary and resolve once the infusion is completed or slowed. Healthcare professionals monitor patients receiving intravenous magnesium sulfate closely, checking vital signs and clinical response regularly.

Patients should be reassured that these common effects, whilst uncomfortable, are generally expected and manageable. However, it remains important to report any concerning symptoms to medical staff promptly.

Serious Side Effects and When to Seek Medical Help

Whilst magnesium sulfate is generally safe when administered correctly, serious adverse effects can occur, particularly with excessive dosing or in vulnerable patients. These complications require immediate medical attention and often relate to magnesium toxicity (hypermagnesaemia).

Serious side effects requiring urgent medical assessment:

• Respiratory depression – reduced breathing rate or shallow breathing, occurring when serum magnesium exceeds approximately 5–6 mmol/L

• Severe hypotension – significant drops in blood pressure causing dizziness, fainting, or collapse

• Cardiac arrhythmias – irregular heartbeat, bradycardia (slow heart rate), or in extreme cases, cardiac arrest (typically at levels ≥7.5 mmol/L)

• Loss of deep tendon reflexes – an early warning sign of magnesium toxicity (around 4–5 mmol/L), routinely monitored in hospital settings

• Profound muscle weakness – difficulty moving or speaking

• Confusion or altered consciousness – progressing to coma in severe toxicity

• Pulmonary oedema – a rare complication, often multifactorial in obstetric patients

Serum magnesium levels correlate with toxicity severity, though clinical monitoring is paramount. This is why regular monitoring of reflexes, respiratory rate, oxygen saturation, and urine output is standard practice during intravenous magnesium therapy.

When to seek immediate help:

In hospital settings, nurses and doctors monitor for complications continuously. Patients should alert staff immediately if experiencing difficulty breathing, chest pain, severe dizziness, loss of consciousness, or inability to move limbs properly.

After discharge, patients should call 999 for severe symptoms such as difficulty breathing or chest pain. For concerning but non-life-threatening symptoms, contact NHS 111 or seek same-day GP advice.

Calcium gluconate is the specific antidote for magnesium toxicity and should be readily available wherever intravenous magnesium sulfate is administered. It works by directly antagonising magnesium's effects on neuromuscular and cardiac tissue. Patients with renal impairment face higher risks as magnesium is primarily excreted by the kidneys, necessitating dose adjustments and enhanced monitoring.

Suspected adverse reactions can be reported via the Yellow Card Scheme at yellowcard.mhra.gov.uk or via the Yellow Card app.

Who Should Avoid Magnesium Sulfate?

Certain patient groups face increased risks from magnesium sulfate therapy, and in some cases, the medication is contraindicated. Healthcare professionals carefully assess individual patient factors before prescribing or administering this treatment.

Contraindications typically include:

• Myasthenia gravis – magnesium sulfate can precipitate myasthenic crisis by further impairing neuromuscular transmission

• Heart block or severe bradycardia – risk of complete heart block or cardiac arrest

• Hypermagnesaemia – existing high magnesium levels contraindicate additional administration

• Known hypersensitivity to magnesium sulfate or its components

Caution required in:

• Renal impairment – magnesium is primarily excreted by the kidneys; severe impairment requires specialist oversight, dose reduction and frequent monitoring of serum magnesium levels

• Respiratory disease – patients with compromised respiratory function are more susceptible to respiratory depression

• Cardiac conduction abnormalities – increased risk of arrhythmias

• Significant hypocalcaemia – may worsen symptoms

• Concurrent use of calcium channel blockers, neuromuscular blocking agents or aminoglycosides – potential for additive effects

In obstetric practice, whilst magnesium sulfate is the treatment of choice for eclampsia prevention, careful consideration is needed regarding timing relative to delivery. Neonates born to mothers receiving magnesium sulfate may exhibit hypotonia (reduced muscle tone), respiratory depression, or lethargy, requiring observation in neonatal units. The MHRA has advised that prolonged or repeated use of magnesium sulfate (>24 hours) in pregnancy may be associated with neonatal bone and electrolyte abnormalities.

Patients should inform healthcare providers about all existing medical conditions, particularly kidney disease, heart problems, or neuromuscular disorders. A comprehensive medication history is essential, as magnesium sulfate interacts with various drugs.

For oral magnesium preparations, patients should be aware that they can reduce the absorption of certain medications including tetracyclines, quinolone antibiotics, bisphosphonates and levothyroxine. These medications should be taken at least 2-3 hours apart from magnesium.

The decision to use magnesium sulfate involves weighing potential benefits against risks. In life-threatening conditions like eclampsia or severe asthma, the benefits typically outweigh risks even in patients with relative contraindications, provided enhanced monitoring is implemented.

Managing and Reducing Side Effects

Effective management of magnesium sulfate side effects relies on careful administration, vigilant monitoring, and prompt intervention when problems arise. Healthcare teams employ several strategies to minimise adverse reactions whilst maintaining therapeutic efficacy.

Clinical monitoring protocols include:

• Regular vital signs assessment – blood pressure, heart rate, respiratory rate, and oxygen saturation according to local protocols (typically at least hourly, with more frequent observations after loading doses)

• Deep tendon reflex testing – typically patellar reflexes, checked regularly as an early marker of toxicity

• Urine output monitoring – ensuring adequate renal function (minimum 25-30 mL/hour)

• Serum magnesium levels – not routinely required for short courses or single doses, but indicated for prolonged therapy, renal impairment, or if toxicity is suspected

• Respiratory function observation – watching for signs of depression

Clear criteria for stopping or reviewing treatment include:

• Absent deep tendon reflexes

• Respiratory rate below 12 breaths per minute

• Urine output below 25-30 mL/hour

Strategies to reduce side effects:

Slowing the infusion rate often alleviates symptoms like flushing, nausea, and dizziness without compromising therapeutic effect. Healthcare professionals may adjust the rate based on patient tolerance. Ensuring adequate hydration helps maintain renal perfusion and magnesium excretion, reducing accumulation risk.

For patients experiencing nausea, antiemetic medications may be prescribed. Reassurance about common sensations like warmth and flushing helps reduce anxiety, which can exacerbate perceived discomfort. Positioning patients comfortably and avoiding sudden movements minimises dizziness related to mild hypotension.

If toxicity develops:

Immediate cessation of magnesium sulfate infusion is the first step. Intravenous calcium gluconate (10 mL of 10% solution) provides rapid reversal of severe toxicity, particularly respiratory or cardiac effects. Supportive measures include oxygen therapy, intravenous fluids, and in severe cases, mechanical ventilation. Senior clinical help should be sought promptly. Patients with renal impairment may require dialysis to remove excess magnesium.

Patient advice:

Whilst most magnesium sulfate administration occurs in hospital under supervision, patients using oral preparations at home should follow dosing instructions precisely, stay well hydrated, and contact their GP if experiencing persistent diarrhoea, severe abdominal pain, or signs of dehydration. For oral magnesium, separate doses from tetracyclines, quinolones, bisphosphonates and levothyroxine by at least 2-3 hours. Understanding that side effects are generally predictable and manageable helps patients feel more comfortable with this important therapeutic intervention.

Frequently Asked Questions