Magnesium Hydroxide

Magnesium hydroxide is a poorly absorbed magnesium salt that acts as an antacid at low doses and as an osmotic laxative at higher doses. Absorbed magnesium is renally excreted, so hypermagnesaemia is the main risk in kidney impairment.

Classification. Antacid; osmotic (saline) laxative.

Mechanism. Neutralises gastric acid to form magnesium chloride and water. In the bowel, unabsorbed magnesium ions retain water osmotically, and magnesium stimulates cholecystokinin release, which increases intestinal secretion and motility.

Pharmacodynamics. Antacid onset within minutes, duration 1 to 3 hours after a meal. Laxative effect within 30 minutes to 6 hours.

Pharmacokinetics. About 15 to 30% of magnesium is absorbed in the small bowel. Absorbed magnesium is excreted by the kidneys; unabsorbed salt is lost in faeces.

Adult oral doses unless stated. Dose by milligrams; check product strength, commonly about 400 mg in 5 mL for single-ingredient suspensions.

Dyspepsia and heartburn.

• Antacid dose 400 to 800 mg (about 5 to 10 mL of a 400 mg in 5 mL suspension), up to four times daily, after meals and at bedtime.

• Usually given combined with aluminium hydroxide to offset diarrhoea.

Constipation, short-term.

• 2.4 to 4.8 g (30 to 60 mL of a 400 mg in 5 mL suspension) daily, as a single dose at bedtime or in divided doses, followed by a full glass of water.

• Use for no more than 7 days without review.

Paediatric constipation (400 mg in 5 mL suspension).

• Age 2 to 5 years: 400 to 1200 mg (5 to 15 mL) daily, single or divided dose.

• Age 6 to 11 years: 1.2 to 2.4 g (15 to 30 mL) daily, single or divided dose.

• Age 12 years and over: adult dosing.

• For childhood functional constipation, macrogol (polyethylene glycol) is first-line.

Contraindications. Severe kidney impairment (CrCl under 30 mL/min); intestinal obstruction, acute abdomen or undiagnosed rectal bleeding; severe dehydration; hypermagnesaemia.

Common adverse effects. Diarrhoea, abdominal cramp and electrolyte loss with laxative doses.

Serious adverse effects.

• Hypermagnesaemia, mainly in kidney impairment, older adults and with prolonged high doses or bowel obstruction. It causes nausea, flushing, hyporeflexia, hypotension, bradycardia, heart block, respiratory depression and coma.

• Dehydration, hypokalaemia and hypernatraemia with repeated laxative use.

Major interactions. Magnesium chelates or adsorbs many drugs; separate dosing.

• Tetracyclines, fluoroquinolones and oral iron: give the other drug 2 hours before or 4 to 6 hours after.

• Integrase inhibitors (dolutegravir, raltegravir, bictegravir): give the antiretroviral 2 hours before or 6 hours after.

• Levothyroxine and bisphosphonates: separate by at least 4 hours and 2 hours respectively.

• Digoxin, ketoconazole, itraconazole, mycophenolate and penicillamine: separate by 2 to 4 hours.

• Enteric-coated tablets may dissolve early.

• Neuromuscular blocking drugs and aminoglycosides: additive weakness if magnesium rises.

Special groups.

• Pregnancy: occasional antacid use is acceptable. For constipation, use fibre first, then lactulose or macrogol.

• Breastfeeding: compatible.

• Children: short-term use only; macrogol is preferred for functional constipation.

• Older adults: risk of hypermagnesaemia with declining kidney function; check eGFR before regular use.

• Myasthenia gravis: hypermagnesaemia worsens weakness; avoid.

• Kidney impairment: avoid regular use when CrCl is under 30 mL/min; use lower doses and monitor magnesium at 30 to 60 mL/min.

• Liver impairment: no dose change; avoid dehydration and electrolyte loss that may precipitate hepatic encephalopathy.

Formulations. Oral suspension, commonly about 400 mg in 5 mL; chewable tablets; combination antacid suspensions and tablets with aluminium hydroxide.

Administration. Shake suspensions well. Antacid doses go after meals and at bedtime. Laxative doses go at bedtime or on waking, followed by a full glass of water.

Baseline. Kidney function before regular or repeated use in older adults and in known kidney disease.

Monitoring.

• Serum magnesium with prolonged use, kidney impairment or new weakness, hypotension or bradycardia.

• Potassium and sodium with repeated laxative use.

• Bowel response; reassess if no effect after 3 days of laxative use.

Therapeutic drug monitoring. Not applicable.

Management of hypermagnesaemia.

• Stop all magnesium.

• Give calcium gluconate 10% 10 mL IV over 5 to 10 minutes for ECG changes, hypotension or respiratory depression, repeated if needed.

• IV fluids with loop diuretic if kidney function allows.

• Haemodialysis for severe toxicity or kidney failure.

Stopping. Stop for diarrhoea, dehydration or rising magnesium. Antacid use for over 2 weeks needs assessment for an underlying cause.

Referral and escalation. Endoscopy for persistent dyspepsia or alarm features. Urgent review for constipation with red flags: weight loss, rectal bleeding, change in bowel habit over age 50 or iron deficiency anaemia.