Generic Name

Magnesium

Mechanism

• NMDA Receptor Antagonist – blocks excitatory glutamate signaling, diminishing neuronal firing and seizures.
• Voltage‑gated Calcium Channel Blocker – reduces intracellular calcium, preventing excessive muscle contraction.
• Endothelial Relaxant – activates guanylate‑cyclase → cGMP → vasodilation; useful in hypertension and pre‑eclampsia.
• Co‑factor for ATP‑Dependent Enzymes – essential for glucose metabolism, phospholipid synthesis, and DNA polymerase activity.

Pharmacokinetics

Kidney function and serum magnesium levels dictate dosing; depletion of competition by alendronate, bisphosphonates, and proton‑pump inhibitors increases risk.

Indications

• Hypomagnesemia → seizures, tremor, arrhythmias.
• Eclampsia/Adverse events → seizure prophylaxis and treatment (MgSO₄).
• Premature labor → tocolysis (IV MgSO₄ infusion).
• Acute coronary syndromes → torsades de pointes prophylaxis.
• Chronic constipation → chronic low‑dose magnesium salts.
• Hypertension → adjunctive therapy (sublingual magnesium citrate).
• Neuro‑ and muscular disorders → myopathy, cramps, or cluster headaches.

Contraindications

• Renal insufficiency (creatinine clearance <30 mL/min) – risk of hypermagnesemia.
• Arachnid venom hypersensitivity or history of anaphylaxis to magnesium salts.
• Acute heart block – magnesium potentiates conduction delay.
• Concomitant sedatives or neuromuscular blockers – additive respiratory depression.
• Pregnancy – use 2nd‑trimester MgSO₄ for eclampsia only; avoid unnecessary parenteral exposure.

*Warnings*
• Hypermagnesemia: signs include hypotension, nystagmus, loss of deep tendon reflexes, respiratory depression.
• Drug interactions: loop diuretics, NSAIDs, amiloride, and some antibiotics (tetracyclines) alter magnesium clearance.

Dosing

*Notes*: Start at lower doses in patients with borderline renal function; monitor serum Mg and renal function closely.

Adverse Effects

Common
• Diarrhea, abdominal cramping (oral doses)
• Flushes, nausea (IV)
• Hypotension, flushing (high‑dose IV)
• Dizziness

Serious
• Hypermagnesemia → muscular weakness, loss of reflexes, respiratory arrest, cardiac arrhythmias.
• Hypotension → shock in severe hypermagnesemia.
• Interactions → additive CNS depression.

Monitoring

• Serum magnesium: baseline, 6–12 h post‑initiation (IV), then daily until stable.
• Renal function: serum creatinine/Clearance at baseline, then weekly (IV).
• ECG: baseline, during IV infusion, look for widened QRS, prolonged PR, or torsades.
• Neuromuscular function: reflexes, respiratory rate, signs of hyporeflexia.
• Volume status: monitor blood pressure, heart rate, urine output.

Clinical Pearls

• Magnesium is often the “forgotten element” – yet it’s vital for >300 enzymatic reactions; consider it whenever you see unexplained loss of reflexes or seizures.
• Co‑administration with bicarbonate can worsen magnesium retention – avoid concomitant use in gout or renal failure patients.
• Sublingual magnesium citrate is an effective non‑parenteral antihypertensive for acute hypertensive emergencies; start 1 g, repeat as needed.
• In eclampsia, the magnesium infusion should be continued 24 h after the last seizure regardless of clinical status; the goal is to keep serum Mg 4–8 mg/dL.
• Intrinsic magnesium deficiency could mask other electrolyte abnormalities – always evaluate K⁺, Ca²⁺, phosphate concurrently.
• Mechanism for constipation relief: magnesium stimulates intestinal water retention and motility via osmotic effect and smooth‑muscle relaxation.
• Check patient’s baseline dietary intake; elderly or patients on proton‑pump inhibitors often have subclinical deficiency; a 250 mg oral supplement may suffice.

Key Takeaway: Magnesium therapy is multifaceted—broader than just a supplement. With careful renal monitoring and titration, magnesium can be a cornerstone in managing seizures, cardiac arrhythmias, hypertension, and more, while minimizing the risk of hypermagnesemia.