Changes in serum magnesium modify digitalis toxicity risk; Mg2+/K+ depletion worsens digoxin arrhythmias, and high-dose MgSO4 shifts electrolyte balance.
Prontuário 3.1.1: "when potassium- and magnesium-depleting drugs (diuretics) are used simultaneously" the likelihood of digitalis toxicity increases; Mg2+ is a cofactor of Na+/K+-ATPase.
Serum Mg2+ and K+, digoxin level, ECG.
Nausea, colour vision changes, extrasystoles, bradycardia.
Monitor Mg2+, K+ and digoxin levels; ECG during the combination; correct potassium depletion.
INFARMED Prontuário Terapêutico (11th ed., 2012), Digitalis 3.1.1 — potassium/magnesium-depleting drugs; DailyMed — Digoxin Tablet label [RLD Lanoxin]: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=dfac7f13-28be-423d-9389-9089da29da17
Magnesium sulphate potentiates the neuromuscular blockade of suxamethonium, potentially prolonging apnoea and respiratory weakness.
Mg2+ inhibits presynaptic acetylcholine release and depresses motor plate excitability, reinforcing the depolarising block (MgSO4 and succinylcholine labels).
Apnoea duration, ventilation, serum Mg2+.
Prolonged apnoea, respiratory weakness after reversal.
In pre-eclampsia treated with MgSO4 and general anaesthesia, inform the anaesthesiologist; have ventilatory support available; consider blockade monitoring.
DailyMed — Magnesium Sulfate Injection label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=5b4b4805-a8ff-4efe-94bc-411090ab5f9c; Succinylcholine Chloride Injection label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=efe5a242-5ba0-4b14-844c-206a79850d81
Magnesium sulphate prolongs and intensifies the non-depolarising neuromuscular blockade of vecuronium.
Mg2+ reduces acetylcholine release and plate sensitivity, increasing curare-blockade duration (labels; obstetric practice with MgSO4).
Tremography/adductor pollicis, ventilation, serum Mg2+.
Prolonged paralysis, failed extubation.
Reduce the initial vecuronium dose in patients on MgSO4; use blockade monitoring (tremography); reversal with neostigmine under supervision.
DailyMed — Vecuronium Bromide Injection and Magnesium Sulfate Injection labels: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=582c7f44-b614-18df-e063-6294a90a74b6
Magnesium sulphate depresses neuromuscular transmission and can blunt reversal with neostigmine.
Mg2+ inhibits presynaptic acetylcholine release, counteracting the effect of cholinesterase inhibition by neostigmine (labels).
Muscle strength (head lift, spontaneous capnography), Mg2+.
Residual weakness, re-intubation, respiratory depression.
Assess reversal clinically and by tremography; correct hypermagnesaemia; ventilatory support until full reversal.
DailyMed — Neostigmine Methylsulfate Injection and Magnesium Sulfate Injection labels: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=4eab6184-1771-4c15-a127-4eb3e47affc2
Calcium is the physiological antidote of magnesium: in MgSO4 toxicity IV calcium gluconate reverses its effects, but mixing them in the same infusion line precipitates.
Direct physiological antagonism: Ca2+ competes with Mg2+ at NMDA channels and acetylcholine release; chemically, Ca2+ forms insoluble salts (MgSO4/gluconate profile: "NEVER mix with sodium bicarbonate"; same rationale for concentrated MgSO4 in the same line).
Patellar reflexes, respiratory rate, serum Mg2+ and Ca2+ (if toxicity).
Muscle weakness, respiratory depression, absent reflexes.
In eclampsia: have IV calcium gluconate available as the MgSO4 antidote; never give in the same line; in chronic oral supplementation, separate doses.
INFARMED Prontuário Terapêutico (11th ed., 2012), Magnesium sulfate entry (calcium antagonism); EMC-UK — Magnesium Sulfate/Calcium Gluconate (documented physiological antagonism in eclampsia): https://www.medicines.org.uk/emc/product/6265/smpc
Furosemide + magnesium: furosemide increases renal magnesium loss.
Loop diuretics (furosemide) increase renal excretion of magnesium (and potassium), potentially causing hypomagnesaemia in patients on prolonged or high-dose treatment. Hypomagnesaemia is clinically relevant in itself (weakness, tetany, arrhythmias) and because it potentiates digitalis toxicity and hypokalaemia. In patients receiving magnesium sulfate (infusion or oral replacement) with furosemide, monitor serum magnesium and potassium, correct deficits and periodically reassess the need for the diuretic and the replacement.
Furosemide + magnesium: furosemide increases renal magnesium loss — risk of hypomagnesaemia.
Loop diuretics increase urinary magnesium excretion; supplementation corrects the deficit but requires monitoring.
Serum magnesium and renal function.
Weakness, cramps, arrhythmias.
Monitor serum magnesium during supplementation in patients on diuretics.
DailyMed (FDA) — approved Furosemide label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=6d9caaab-d874-4cf1-b9fe-408452a18998 ; approved Magnesium sulfate label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=a5a9f565-639c-4b22-b9e5-718bac7cfcf3 — additional reference: Prontuário Terapêutico do INFARMED (11th ed., 2012)
Aminoglycoside + magnesium: risk of neuromuscular blockade and additive nephrotoxicity.
Aminoglycosides (gentamicin) can cause neuromuscular blockade, especially in patients with myasthenia gravis, botulism or hypocalcaemia; magnesium at high concentrations also depresses neuromuscular transmission. Together, the risk of muscle weakness and respiratory depression increases. In addition, both can damage the kidney: nephrotoxicity is additive, and impaired renal function reduces clearance of both, raising levels even further. In patients receiving gentamicin and IV magnesium sulfate (e.g., pre-eclampsia with infection), monitor renal function, serum magnesium and aminoglycoside levels, and watch muscle strength and ventilation.
Aminoglycoside + magnesium: risk of neuromuscular blockade and additive nephrotoxicity.
Magnesium sulfate may potentiate aminoglycoside neuromuscular blockade; both may affect renal function.
Renal function, magnesium, neurological signs.
Weakness, respiratory depression, rising creatinine.
Monitor renal function and signs of muscle weakness during co-administration.
DailyMed (FDA) — approved Gentamicin label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=977180b3-a222-4282-d485-4a3217674305 ; approved Magnesium sulfate label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=a5a9f565-639c-4b22-b9e5-718bac7cfcf3 — additional reference: Prontuário Terapêutico do INFARMED (11th ed., 2012)
Ciprofloxacin + magnesium: chelation reduces fluoroquinolone absorption.
Magnesium, a divalent cation, forms insoluble chelates with fluoroquinolones in the gastrointestinal tract, reducing the oral bioavailability of ciprofloxacin and the risk of therapeutic failure in serious infections. The ciprofloxacin label recommends giving the antibiotic 2 hours before or 6 hours after magnesium-containing antacids or supplements (including magnesium laxatives). In hospitalised patients receiving oral/enteral magnesium sulfate and ciprofloxacin, check the administration times of both to ensure antibacterial efficacy.
Magnesium + ciprofloxacin: chelation with reduced absorption. Separate by 2–6 h.
Magnesium cations chelate ciprofloxacin in the gut.
Clinical response to antibiotic therapy.
Therapy failure.
Separate oral dosing by at least 2 hours.
DailyMed (FDA) — approved Ciprofloxacin label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=c527d138-e32c-418f-9573-a3d8a796279f ; approved Magnesium sulfate label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=a5a9f565-639c-4b22-b9e5-718bac7cfcf3 — additional reference: Prontuário Terapêutico do INFARMED (11th ed., 2012)
Salbutamol + magnesium: additive hypokalaemia (the β2-agonist lowers potassium).
β2-agonists such as salbutamol activate the Na⁺/K⁺-ATPase pump and promote intracellular potassium uptake, causing transient hypokalaemia. Magnesium sulfate can also lower serum potassium and, at high concentrations, depresses neuromuscular and cardiac conduction. Together (a common situation in severe acute asthma, where both are used), hypokalaemia can be more pronounced and increase the risk of arrhythmias, especially in patients with cardiovascular disease or on digoxin. Serum potassium and ECG should be monitored during magnesium infusion in patients receiving high-dose β2-agonists, correcting hypokalaemia if needed.
Salbutamol + magnesium: additive hypokalaemia (the β2-agonist lowers potassium). Monitor serum potassium and ECG.
β2-agonists cause K+ shift into cells; combined with magnesium, hypokalaemia may be accentuated, especially at high doses.
Serum potassium.
Weakness, cramps, arrhythmias.
Monitor potassium at high doses or with prolonged use.
DailyMed (FDA) — approved Salbutamol label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=3236f82c-14e9-dfd4-e063-6294a90ac43d ; approved Magnesium sulfate label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=a5a9f565-639c-4b22-b9e5-718bac7cfcf3 — additional reference: Prontuário Terapêutico do INFARMED (11th ed., 2012)
Alcohol increases renal magnesium excretion and worsens the osmotic diarrhoea of oral magnesium sulfate — additional electrolyte loss and dehydration.
Avoid alcohol during oral magnesium sulfate treatment (laxative).
Prontuário Terapêutico INFARMED, Annex 6 (alcohol — dehydration and electrolytes); MgSO4 pharmacology.
Magnesium is exclusively renally excreted — in chronic kidney disease it accumulates causing CNS depression, respiratory arrest and heart block.
Reduce dose and monitor serum magnesium and patellar reflexes at any degree of renal impairment.
Prontuário Terapêutico INFARMED, 7.2 Uterotonics (MgSO4): "...monitoring of plasma magnesium levels"; renal excretion.
Magnesium accumulates in renal impairment, risking hypermagnesaemia.
Use with caution and monitor serum magnesium.
EMC-UK (MHRA) — approved Magnesium sulfate SmPC: https://www.medicines.org.uk/emc/product/10502/smpc
Magnesium inhibits acetylcholine release at the neuromuscular junction — it can trigger a severe myasthenic crisis.
Avoid IV magnesium in myasthenia; if unavoidable, ensure ventilatory support is available.
EMC-UK — Magnesium sulfate injection SmPC § 4.3 (myasthenia gravis); Prontuário (neuromuscular mechanism).
Avoid in severe hepatic impairment.
Avoid use in severe hepatic impairment.
Prontuário Terapêutico do INFARMED (11th ed., 2012) — Annex 3, Drugs and hepatic impairment: Avoid in severe hepatic impairment.
IV magnesium sulfate is the anticonvulsant and neuroprotective agent of choice in severe pre-eclampsia/eclampsia and for fetal neuroprotection in threatened preterm labour (<32 weeks); widely used in obstetrics with a well-established fetal safety profile. At prolonged tocolytic doses it may be associated with neonatal hypotonia, respiratory depression and hypocalcaemia.
Continued use in obstetrics is standard practice; administer under maternal monitoring of reflexes, respiratory rate and urine output (magnesium toxicity). The Prontuário notes its tocolytic role and use "with caution in liver disease" (7.2.3).
Excreted into milk in small amounts; no reports of adverse effects in infants at usual obstetric doses — compatible.
Prontuário Terapêutico INFARMED — 7.2.3: "magnesium salts (magnesium sulfate) are also effective as tocolytics... Should be used with caution in liver disease" (l. 24080-90); EMC-UK (MHRA) — Magnesium Sulfate Injection SmPC.
Clinical and educational support tool. The information does not replace a medical prescription or the opinion of a qualified healthcare professional.
Anticonvulsant in eclampsia (↓ seizures by ~ 50% vs. diazepam/phenytoin); bronchodilation in severe asthma; antiarrhythmic in torsades.
Physiological Ca2+ antagonist: blocks NMDA channels (neuroprotection) and voltage-gated Ca2+ channels (vasodilation, bronchodilation); stabilises membrane potential.
IV: 100%; IM: ~ 90%; oral: poor absorption (osmotic laxative effect).
No metabolism; rapid renal excretion (half-life kidney-function dependent).
~ 4–6 h (normal kidneys).
Medicines from the same therapeutic group (ATC classification) or the same pharmacological class.
