What is the recommended dose of Milk of Magnesia (magnesium hydroxide) for a patient with constipation or heartburn, considering their medical history and potential renal impairment?

Milk of Magnesia Dosing for Constipation and Heartburn

For constipation, start with 30-60 mL (2,400-4,800 mg magnesium hydroxide) as a single bedtime dose with a full glass of water, but avoid use entirely in patients with renal impairment due to potentially fatal hypermagnesemia risk. 1

Dosing by Indication

For Constipation (Primary Use)

• Initial dose: 30 mL (2,400 mg magnesium hydroxide) taken once daily, preferably at bedtime 1
• Maximum dose: 60 mL (4,800 mg magnesium hydroxide) per 24-hour period 1
• Administration: Take with a full 8 oz glass of liquid 1
• Duration: While FDA labeling doesn't specify duration, clinical trials support 4-week use with longer-term use likely appropriate 2, 3

Alternative Magnesium Formulation (Magnesium Oxide)

• Initial dose: 400-500 mg daily 2, 4
• Studied effective dose: 1,000-1,500 mg daily in divided doses 2, 4
• Titration: Increase based on symptom response and tolerability 2
• No clear maximum dose, but titrate to effect 2

Critical Safety Considerations

Absolute Contraindications

• Renal impairment (creatinine clearance <20 mL/dL): Risk of fatal hypermagnesemia even with standard doses 4, 5, 6
• Multiple case reports document fatal hypermagnesemia in patients taking magnesium hydroxide 3,000 mg daily, even in those with initially normal renal function 5, 7, 8, 6

High-Risk Scenarios Requiring Extreme Caution

• Bowel obstruction or severe constipation: Creates a reservoir for continuous magnesium absorption, leading to life-threatening hypermagnesemia 7, 8, 6
• Normal renal function does NOT protect against toxicity when gastrointestinal motility is impaired 7, 8, 6
• Elderly patients: The case of an 82-year-old woman developing metabolic encephalopathy from 3,000 mg daily highlights age-related vulnerability 5

Monitoring Requirements

• Regular magnesium level monitoring is essential, especially with prolonged use or in any patient with gastrointestinal dysmotility 5, 6
• Signs of hypermagnesemia: Lethargy, hypotension, prolonged QT interval, AV block, metabolic encephalopathy 5, 7, 8
• Cardiac arrest can occur even with prompt initiation of renal replacement therapy 5, 6

Treatment Algorithm for Constipation

First-Line Approach

• Fiber supplementation (14 g/1,000 kcal intake) with adequate hydration for mild constipation 2
• Polyethylene glycol 17 g daily is preferred over magnesium-based laxatives due to superior safety profile and durable 6-month response 2, 3

Second-Line Options

• Magnesium hydroxide or magnesium oxide can be used if PEG is not tolerated or as adjunct to fiber 3, 4
• The 2023 AGA-ACG guidelines give a conditional recommendation (not strong) for magnesium oxide based on very low quality evidence 2

Rescue Therapy

• Bisacodyl 5-10 mg daily for short-term use (≤4 weeks) or occasional rescue 2
• If constipation persists after 4 weeks, consider switching to PEG or adding stimulant laxative 3

Common Pitfalls to Avoid

• Do not assume normal renal function provides safety: Fatal cases occurred in patients with creatinine 0.9 mg/dL 7
• Do not use in patients with abdominal pain or distension without ruling out obstruction 7, 8
• Do not exceed 60 mL per 24 hours of magnesium hydroxide 1
• Do not use as first-line therapy: PEG has stronger evidence and better safety profile 2
• Pregnancy: Use with caution; lactulose is the only osmotic agent specifically studied in pregnancy 2, 4

Mechanism and Expected Effects

• Osmotic laxative: Draws water into intestinal lumen to soften stool and stimulate bowel movements 3
• Common side effects: Dose-dependent bloating, flatulence, diarrhea (11-37% of patients) 4
• Onset: Typically works within 30 minutes to 6 hours 9