Magnesium oxide
Magnesium oxide (MgO)
Generic Name
Magnesium oxide (MgO)
Mechanism
- Mg²⁺ Release – Dissolution of MgO in the acidic gastric milieu liberates magnesium ions that:
- Stabilize gastric mucosa by forming a protective coating on the stomach lining.
- Neutralize gastric acid (pH‑dependent alkali) and raise intragastric pH.
- Act as an osmotic laxative by attracting water into the colon, increasing stool volume and accelerating transit.
- Inhibit voltage‑gated calcium channels in smooth muscle, thereby relaxing intestinal motility in constipation and reducing cardiac excitability in hypomagnesemia.
Pharmacokinetics
| Parameter | Description |
| Absorption | Poor across the GI tract; efflux transporters (MRP2) limit uptake. Uptake occurs mostly in the ileum and proximal jejunum. |
| Bioavailability | 400 mg elemental Mg). |
| Distribution | Primarily extracellular fluid, with ~1 % extraskeletal. Low protein binding. |
| Metabolism | Non‑enzymatic; no active metabolites. |
| Elimination | Renal excretion of Mg²⁺ (≈ 70 %); ~30 % fecal loss via urinary excretion and intestinal secretion. Half‑life ~6‑8 h in healthy adults. |
| Drug Interactions | Calcium carbonate, antacids, PPIs, certain antibiotics (e.g., ciprofloxacin) may reduce MgO absorption. Diuretics & ACE inhibitors may potentiate hypomagnesemia. |
Indications
- Acid‑related disorders: heartburn, gastro‑oesophageal reflux disease, peptic ulcer disease, postoperative pain.
- Constipation: osmotic laxative in mild to moderate constipation.
- Magnesium deficiency: supplementation in hypomagnesemia, preeclampsia prophylaxis, or postoperative magnesium depletion.
- Preoperative prophylaxis: reduction of non‑osteonecrosis magnesium loss in certain surgeries.
Contraindications
- Absolute contraindications: known hypersensitivity to magnesium compounds; severe renal impairment (CrCl <30 mL/min); ileal obstruction; severe diarrhoea with rapid loss of Mg²⁺.
- Relative contraindications: cardiac pacemakers (conduction abnormalities), myasthenia gravis, dexmedetomidine use.
- Warnings:
- Monitor Mg²⁺ in patients on diuretics or ACE inhibitors to avoid hypomagnesemia.
- Avoid unnecessary high-dose MgO in patients with underlying hypermagnesemia (e.g., organ transplant, eclampsia).
- Be cautious in elderly patients due to decreased renal clearance.
Dosing
| Condition | Typical Dose | Duration | Notes |
| Heartburn / GERD | 400–800 mg elemental Mg (1–2 tabs) 2–4×/day | As needed | Take after meals; avoid with H2 blockers for optimal pH raise. |
| Constipation | 400–800 mg elemental Mg (1–2 tabs) orally | 1–7 days | Increase dose with progressive response. |
| Magnesium deficiency | 500–1000 mg elemental Mg (1–2 tabs) daily | 2–6 weeks | Recheck serum Mg after 2–4 weeks. |
| Preeclampsia prophylaxis | 200 mg elemental Mg (1 tab) 3×/day | Pre‑eclamptic period | Use 0.5 g elemental Mg per module as per local protocol. |
• Formulations: tablets, chewable tablets, oral suspension (5 % magnesium salt) enhance tolerability.
• Administration: With water; avoid taking simultaneously with calcium carbonate or high‑dose antacids unless gastro‑protective plan is in place.
Adverse Effects
- Common:
- Diarrhoea, loose stools, abdominal cramping (osmotic laxative effect).
- Nausea, vomiting (dose‑related).
- Flatulence; mild metallic taste.
- Serious:
- Hypotension (in high doses or IV infusion).
- Cardiac arrhythmias in severe hypermagnesemia.
- Respiratory depression at supratherapeutic levels.
- Fecal incontinence due to excessive laxation.
Monitoring
- Serum magnesium: baseline, 2–4 weeks during supplementation for deficiency, and after any renal function decline.
- Renal function: serum creatinine, eGFR within 2–4 weeks of therapy initiation.
- Cardiac assessment: ECG in patients with underlying conduction disease or at risk of arrhythmias (> 10 mg/kg of elemental Mg).
- Electrolytes: potassium, calcium; hypermagnesemia can mask hypokalemia or hypocalcemia.
Clinical Pearls
- Use MgO sparingly as antacid: Choose magnesium succinate or citrate for milder acid reflux; MgO offers less rapid symptom relief but higher elemental Mg dose.
- Fixed‑dose combinations: MgO + calcium carbonate often used in over‑the‑counter antacids; beware of calcium‑magnesium antagonism and potential GI upset.
- “Gentle” laxative strategy: Start at the lowest effective dose and titrate upward; avoid exceeding 800 mg elemental Mg/day in chronic constipation to prevent excessive diarrhoea.
- Preeclampsia and Magnesium Therapy: MgO is not the preferred intravenous agent for seizure prophylaxis; magnesium sulfate is standard. Oral MgO is sometimes used adjunctively for “maintenance” magnesium levels.
- Kidney‑protected dosing: In patients with mild CKD (CrCl 30–60 mL/min), reduce dose by 30 % and extend monitoring intervals.
- Drug‑drug interferences: Concomitant use of PPIs may *reduce* MgO absorption; coordinate timing or consider alternative antacids for acid control.
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• *For further reading: R. L. Lundberg, “Magnesium in Human Health and Disease”, *J. Clin. Pharm.*, 2023.*