What is the recommended dose of magnesium hydroxide (Milk of Magnesia) for adults and children, including formulation-specific dosing and contraindications?

Milk of Magnesia Dosing

For adults and children over 12 years, the recommended dose of milk of magnesia (magnesium hydroxide) is 30-60 mL (2,400-4,800 mg) once daily, preferably at bedtime, with a full glass of water. 1

Adult Dosing

• Standard dose: 30 mL (approximately 1 oz or 2,400 mg magnesium hydroxide) to 60 mL once daily, preferably at bedtime 1
• The dose may be taken once daily or divided throughout the day as directed by a physician 1
• Always drink a full 8 oz glass of liquid with each dose 1
• Do not exceed the maximum recommended daily dose in a 24-hour period 1

Pediatric Dosing

• Children over 12 years: Same as adult dosing (30-60 mL once daily) 1
• Children under 12 years: Consult a physician before use 1
• In a randomized trial of children with chronic constipation, milk of magnesia was effective and safe for long-term treatment (12 months), though compliance was lower (65%) compared to polyethylene glycol (95%) 2

Clinical Context and Efficacy

Milk of magnesia works as an osmotic laxative by drawing water into the intestines, making stool softer and easier to pass 3. The typical dosing of approximately 30 mL (1 oz) twice daily is common, though once-daily dosing at bedtime is often sufficient 3. The American Gastroenterological Association conditionally recommends magnesium-based laxatives for chronic idiopathic constipation, though the evidence quality is very low 3.

Critical Safety Considerations

Absolute Contraindications

Avoid milk of magnesia entirely in patients with creatinine clearance <20 mL/min due to the high risk of life-threatening hypermagnesemia. 3, 4

• The kidneys are responsible for nearly all magnesium excretion, and impaired renal function prevents adequate elimination 4
• Severe hypermagnesemia can manifest with bradycardia, asystole, shock, hypothermia, and respiratory failure 5
• Fatal cases have been reported in patients with end-stage renal disease who used milk of magnesia 5, 6

High-Risk Populations Requiring Caution

• Elderly patients: Use with extreme caution due to increased risk of hypermagnesemia and electrolyte disturbances 3
• Patients with cardiac or renal comorbidities: Individualize laxative choice, particularly those on diuretics or cardiac glycosides 3
• Non-ambulatory patients with low fluid intake: Increased risk of complications 3
• Pregnant women: Use only under medical supervision 3

Monitoring Requirements

• Check creatinine clearance before prescribing any magnesium-containing laxative 3
• If creatinine clearance is >60 mL/min, magnesium-containing laxatives may be used with standard precautions, but monitor for declining renal function 3
• Use alternative laxatives such as polyethylene glycol (PEG), lactulose, or stimulant laxatives (bisacodyl, senna) in patients with renal impairment 3

Alternative Laxatives for High-Risk Patients

Polyethylene glycol (PEG) 17g daily is the preferred osmotic laxative in patients with chronic kidney disease, as it has no systemic absorption concerns and demonstrates durable 6-month response. 3

• Stimulant laxatives (bisacodyl, senna) can be used short-term or as rescue therapy without renal concerns 3
• In the pediatric trial, PEG and milk of magnesia were equally effective, but PEG was better accepted and had higher compliance 2

Common Pitfalls

• Never assume normal serum magnesium excludes toxicity risk: Less than 1% of total body magnesium is in blood, and patients with renal impairment can accumulate magnesium despite normal initial levels 4
• Failing to check renal function before prescribing: This is the most critical error, as hypermagnesemia in renal failure can be fatal 3, 5
• Using in elderly cancer patients: The European Society for Medical Oncology recommends against saline laxatives including magnesium hydroxide in this population 3