What is the recommended dosage of magnesium hydroxide (Milk of Magnesia) for treating constipation in pediatric patients?

Magnesium Hydroxide (Milk of Magnesia) Dosing for Pediatric Constipation

For children with functional constipation, magnesium hydroxide is dosed at 1-2 mL/kg/day (or approximately 400-1200 mg/day depending on age), with the FDA-approved adult formulation recommending 30-60 mL for children over 12 years, while younger children require physician-directed dosing. 1

Age-Specific Dosing Algorithm

Children Under 12 Years

• Consult a physician for specific dosing as the FDA label does not provide standardized pediatric doses for this age group 1
• Research supports typical dosing of 400-800 mg/day (median 600 mg/day) for children aged 1-14 years with functional constipation 2
• Studies have used magnesium hydroxide successfully in children as young as 6 months of age 3

Children 12 Years and Older

• 30-60 mL (one to two unit doses) once daily, preferably at bedtime 1
• Must drink a full 8 oz glass of liquid with each dose 1
• Do not exceed maximum recommended daily dose in 24 hours 1

Treatment Approach and Duration

Start with disimpaction if fecal impaction is present, followed by daily maintenance therapy rather than intermittent use, as aggressive initial treatment predicts better outcomes 4

• Treatment duration typically extends 6-12 months for functional constipation 3, 5
• Success is defined as ≥3 bowel movements per week without fecal incontinence, impaction, abdominal pain, or need for additional laxatives 3
• Magnesium hydroxide works through osmotic mechanisms, drawing water into the intestinal lumen 6

Comparative Efficacy

Magnesium hydroxide demonstrates equivalent efficacy to polyethylene glycol (PEG) 3350 in treating pediatric functional constipation, though acceptance differs by age 3, 5

• Both medications improve stool consistency, bowel movement frequency, fecal incontinence, abdominal pain, and straining with no significant differences 5
• PEG 3350 shows better acceptance in children over 4 years of age (42.9% refused magnesium hydroxide vs 0% refused PEG) due to being odorless and tasteless 5
• Magnesium hydroxide can be considered first-line treatment for children under 4 years of age where acceptance is comparable 3

Critical Safety Considerations

Contraindications and Precautions

• Absolutely contraindicated in patients with renal insufficiency due to risk of hypermagnesemia 7, 6
• Rule out bowel obstruction before initiating therapy using physical exam and abdominal x-ray if clinically indicated 6
• Avoid in patients with abdominal pain, nausea, or vomiting of unknown etiology 6
• Contraindicated in gastrointestinal diseases (ileus, ischemic colitis) where hypermagnesemia risk increases even with normal renal function 6

Monitoring Requirements

• Serum magnesium levels increase significantly but not critically in children with normal renal function taking daily magnesium oxide 2
• In a study of 120 children, median serum magnesium was 2.4 mg/dL (range up to 3.2 mg/dL) compared to 2.2 mg/dL in controls, with no clinical hypermagnesemia symptoms 2
• Renal magnesium clearance increases compensatorily in children with normal kidney function 2
• Serum magnesium concentration decreases with age, suggesting younger children may have higher levels 2

Common Pitfalls to Avoid

Primary care physicians tend to undertreat childhood constipation, with nearly 40% of children remaining symptomatic after 2 months when treatment is insufficiently aggressive 4

• Do not prescribe fixed doses without clear instructions to titrate - only 5% of physicians in one study instructed parents to adjust dosing based on response 4
• Do not use intermittent or "as needed" dosing - daily maintenance therapy is essential 4
• Do not skip disimpaction when fecal impaction is present - children who underwent colonic evacuation followed by daily laxatives had significantly better outcomes 4
• Ensure adequate hydration during treatment to minimize hypermagnesemia risk 6

Alternative and Adjunctive Therapy

If constipation persists after 4 weeks of magnesium hydroxide:

• Add a stimulant laxative (bisacodyl 10-15 mg daily) 6
• Switch to polyethylene glycol 17 g daily, which has demonstrated durable 6-month response 6
• Consider lactulose (30-60 mL twice to four times daily) or sorbitol as alternative osmotic agents 6
• Combine with fiber supplementation and behavioral modification for comprehensive management 8