What are the uses and precautions of milk of magnesia (magnesium hydroxide)?

Milk of Magnesia: Uses and Precautions

Milk of magnesia (magnesium hydroxide) is an effective and inexpensive osmotic laxative for constipation management, typically dosed at 30 mL (approximately 1 oz) at bedtime, but must be avoided in patients with significant renal impairment (creatinine clearance <20 mL/min) due to risk of life-threatening hypermagnesemia. 1, 2

Primary Clinical Uses

• Constipation treatment: Milk of magnesia works by drawing water into the intestines through osmotic action, softening stool and stimulating bowel movements 1
• Cost-effective option: Daily treatment costs approximately $1 or less, making it an accessible first-line osmotic agent 1
• Guideline-endorsed therapy: Osmotic laxatives including magnesium salts are strongly endorsed by ESMO and other major guidelines for chronic constipation, particularly in advanced disease 3, 1

Dosing and Administration

• Standard dosing: 30 mL (2,400 mg) at bedtime is the most common and often sufficient regimen 1
• Alternative regimen: 30 mL twice daily can be used if once-daily dosing is inadequate 1
• Treatment algorithm: Start with dietary modifications and increased fiber intake first, then add milk of magnesia at bedtime if insufficient response, and consider adding a stimulant laxative or switching to PEG if constipation remains unresolved 1

Critical Safety Precautions and Contraindications

Renal Impairment (Most Important)

• Absolute contraindication: Avoid use in patients with creatinine clearance <20 mL/min due to risk of hypermagnesemia 1, 2
• Use with extreme caution: Even patients with moderate renal insufficiency require careful monitoring, as excessive doses can lead to hypermagnesemia 3, 1
• Fatal cases reported: Severe hypermagnesemia from magnesium hydroxide has caused death even in patients with initially normal renal function when combined with bowel obstruction 4

Other Important Warnings

• Kidney disease: Ask a doctor before use if patient has any kidney disease 2
• Magnesium-restricted diet: Contraindicated in patients on magnesium restriction 2
• Bowel obstruction risk: Patients with bowel obstruction or impaired GI motility are at dramatically increased risk of hypermagnesemia, as magnesium absorption increases when transit time is prolonged 4
• Duration limit: Stop and consult a doctor if laxative use exceeds 1 week without resolution 2
• Red flag symptoms: Discontinue immediately if rectal bleeding or no bowel movement occurs after use, as these may indicate serious conditions 2

Special Populations

• Elderly patients: Use with heightened caution due to increased risk of electrolyte disturbances and higher likelihood of underlying renal impairment 1
• Pregnancy and breastfeeding: Should only be used under medical supervision; lactulose is the only osmotic agent specifically studied in pregnancy 1, 2
• Children: Milk of magnesia is effective for pediatric constipation, though PEG may be better tolerated with higher compliance rates (95% vs 65%) 5, 6

Comparative Effectiveness

• PEG superiority: Polyethylene glycol (PEG) demonstrates slightly greater efficacy than milk of magnesia (0.69 more stools per week), though this difference may not be clinically significant 7, 6
• Better than lactulose: Milk of magnesia shows superior efficacy compared to lactulose in head-to-head trials 6
• Liquid paraffin alternative: Mineral oil shows comparable or superior efficacy but carries risk of aspiration pneumonia 3

Common Adverse Effects

• Gastrointestinal: Diarrhea, abdominal pain, nausea, flatulence, and abdominal distention are the most common side effects 3, 5, 7
• Taste intolerance: Some patients cannot tolerate the taste, leading to poor compliance 5
• Electrolyte disturbances: Beyond hypermagnesemia, excessive use can cause other electrolyte imbalances 1

Clinical Pitfalls to Avoid

• Overlooking renal function: Always check renal function before initiating therapy, as even mild renal impairment increases risk 1, 2
• Ignoring bowel obstruction: The combination of magnesium hydroxide and bowel obstruction or severe constipation with impaired motility can be lethal due to increased magnesium absorption 4
• Prolonged use without reassessment: If constipation persists beyond 1 week, reevaluate the diagnosis and consider alternative agents or additional therapies 2
• Fecal impaction with overflow: When constipation presents with diarrhea, consider fecal impaction with overflow diarrhea, which requires rectal disimpaction rather than oral laxatives 1

When to Escalate or Switch Therapy

• After 4 weeks of inadequate response: Add a stimulant laxative (senna, bisacodyl) or switch to PEG 1
• Suspected gastroparesis: Consider prokinetic agents rather than continuing osmotic therapy 8
• Rectal examination findings: If digital rectal exam reveals fecal impaction, suppositories or enemas are preferred first-line therapy over oral agents 3