Educational use only. Drug cards are AI-assisted study material for NCLEX preparation.
Magnesium sulfate (MgSO4)
Mechanism of action
Replaces magnesium and competes with calcium at neuromuscular and vascular smooth-muscle calcium channels — producing CNS depression, smooth-muscle relaxation, and anticonvulsant activity. Used for seizure prophylaxis and treatment in pre-eclampsia/eclampsia, severe asthma exacerbation refractory to bronchodilators, torsades de pointes, and hypomagnesemia replacement.
Adverse effects
Life-threatening / NCLEX-tested
- Magnesium toxicity — therapeutic 4–7 mEq/L for OB; loss of deep tendon reflexes is the FIRST WARNING (DTRs disappear at ~10), respiratory depression at ~12, cardiac arrest at ~15+
- Severe respiratory depression
- Hypotension (often profound with rapid IV loading)
- Cardiac arrest
- Pulmonary edema (especially in obstetric patients with high cumulative volume)
- Decreased fetal heart rate variability and neonatal respiratory depression
Side effects
Common — what to teach
- Flushing, warmth (very common during IV bolus — warn the patient)
- Sweating, drowsiness
- Nausea
- Mild muscle weakness
- Burning at IV site
Food & drug interactions
Calcium channel blockers compound hypotension. Neuromuscular blockers prolong paralysis dramatically — the OB nurse must alert anesthesia before delivery if a Mg infusion is running. Aminoglycosides plus magnesium can cause synergistic neuromuscular blockade.
Nursing implications
Assessment, monitoring, patient teaching
- Monitor on a Mg sulfate protocol: deep tendon reflexes (DTRs) every 1–2 hours, RR (≥ 12), urine output (≥ 30 mL/hr — magnesium is renally cleared and accumulates in oliguria), and serum Mg level per protocol
- Have CALCIUM GLUCONATE (or calcium chloride) at the bedside as the antidote — 1 g IV slowly for severe toxicity
- Loading dose: typical OB load 4–6 g IV over 15–20 min, then 1–2 g/hr maintenance — always via infusion pump, never gravity
- Two-nurse independent verification of pump settings
- Monitor fetal heart rate continuously in OB use; alert the team to any decrease in FHR variability
- Strict I&O — oliguria is the most common driver of toxicity
- Have intubation equipment available for severe pre-eclampsia/eclampsia
When to hold / contraindications
- Loss of deep tendon reflexes
- RR < 12
- Urine output < 30 mL/hr (magnesium accumulates)
- Serum Mg level above protocol target
- Heart block or significant bradycardia
- Myasthenia gravis (relative contraindication)
Memory anchor
Magnesium toxicity goes in this order: "DTRs gone → RR down → BP crash → arrest." Antidote is CALCIUM GLUCONATE at the bedside. Watch the urine output.
Practice Magnesium questions
Test your recall on real NCLEX-style pharmacology questions — with full rationales and an AI Coach for the parts you miss.