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

Milk of Magnesia (Magnesium Hydroxide): Uses and Precautions

Milk of magnesia is an effective osmotic laxative for constipation management that works by drawing water into the intestines to soften stool, but should be avoided in patients with renal impairment due to risk of hypermagnesemia. 1

Primary Uses

• Treatment of Constipation: Functions as an osmotic laxative by drawing water into the intestines, making stool softer and easier to pass 1, 2
• Antacid: Can neutralize stomach acid due to its alkaline properties 2

Dosing Guidelines

• Adults and children over 12 years: 30-60 mL (2400-4800 mg) once daily, preferably at bedtime 3
• Children under 12 years: Consult a doctor for appropriate dosing 3
• Administration: Shake well before use and drink a full glass (8 oz) of water with each dose 3

Efficacy for Constipation

• Conditionally recommended by the American Gastroenterological Association for chronic idiopathic constipation based on very low-quality evidence 4
• Often used after increasing fiber intake as part of a gradual approach to treating constipation 1
• Meta-analysis showed PEG (polyethylene glycol) produces slightly more stools per week than milk of magnesia, though the clinical significance of this difference may be small 5

Algorithm for Constipation Management

1. Start with dietary modifications and increased fiber intake
2. If insufficient response, add milk of magnesia at 30 mL at bedtime
3. Consider adding a stimulant laxative or switching to PEG if constipation remains unresolved 1

Precautions and Contraindications

• Renal Impairment: Strictly avoid in patients with significant renal dysfunction (creatinine clearance <20 mL/min) due to risk of potentially fatal hypermagnesemia 1, 3, 6
• Bowel Obstruction: Contraindicated in intestinal obstruction as it may worsen the condition and increase risk of magnesium toxicity 7
• Prolonged Use: Should not be used for more than one week without medical supervision 3
• Drug Interactions: May interact with certain prescription medications; consult healthcare provider before use 3

Warning Signs to Discontinue Use

• Rectal bleeding
• No bowel movement after using the product
• Severe abdominal pain 3

Special Populations

• Elderly: Use with caution due to increased risk of electrolyte disturbances and higher likelihood of reduced renal function 1
• Pregnancy and Breastfeeding: Consult healthcare provider before use 3

Potential Adverse Effects

• Common: Abdominal discomfort, diarrhea, nausea 2
• Severe (rare): Hypermagnesemia, which can manifest as hypotension, bradycardia, respiratory depression, and cardiac arrhythmias, particularly in patients with renal impairment 7, 6

Cost Considerations

• Inexpensive osmotic agent for constipation management, with approximate daily cost of $1 or less 1