What is the standard dose of milk of magnesia (magnesium hydroxide) for constipation?

Standard Dose of Milk of Magnesia for Constipation

The standard dose of milk of magnesia (magnesium hydroxide) for constipation in adults and children over 12 years of age is 30-60 mL (containing 2,400 mg of magnesium hydroxide per 30 mL), preferably taken at bedtime with a full glass (8 oz) of water. 1

Dosing Guidelines

• Adults and children over 12 years: 30-60 mL once daily, preferably at bedtime 1
• Children under 12 years: Consult a doctor for appropriate dosing 1
• Each 30 mL (1 unit dose) contains 2,400 mg of magnesium hydroxide 1
• Milk of magnesia should be taken with a full glass (8 oz) of liquid to enhance effectiveness 1
• Shake the bottle well before use to ensure proper distribution of the medication 1

Clinical Considerations

• Milk of magnesia is an osmotic laxative that works by drawing water into the intestines, making stool softer and easier to pass 2
• It can be used as an inexpensive osmotic agent for constipation management, with an approximate daily cost of $1 or less 2
• The American Gastroenterological Association suggests milk of magnesia as part of a gradual approach to treating constipation, often after increasing fiber intake 2
• Typical dosing is approximately 1 oz (30 mL) twice daily, though once-daily dosing at bedtime is common and often sufficient 2

Efficacy and Comparative Effectiveness

• Magnesium-based laxatives are conditionally recommended for chronic idiopathic constipation based on very low-quality evidence 2
• PEG (polyethylene glycol) has been shown to be slightly more effective than milk of magnesia for constipation management, with a mean difference of 0.69 more stools per week 3
• In pediatric studies, milk of magnesia and PEG were equally effective in long-term treatment of constipation, though PEG had better acceptance rates (95% vs 65%) 4

Safety Considerations and Precautions

• Avoid use in patients with significant renal impairment (creatinine clearance <20 mL/min) due to risk of hypermagnesemia 2, 5
• Severe hypermagnesemia can occur even in patients with normal renal function if they have bowel obstruction or severe constipation with ileus 6, 7, 8
• Monitor for signs of hypermagnesemia, which include lethargy, hypotension, and cardiac conduction abnormalities 7, 8
• Do not exceed the recommended dose within a 24-hour period 1

Algorithm for Use in Constipation Management

1. Start with dietary modifications and increased fiber intake 2
2. If insufficient response, add milk of magnesia at 30 mL (2,400 mg) at bedtime 2, 1
3. If needed, increase to 60 mL (4,800 mg) at bedtime 1
4. For persistent constipation, consider adding a stimulant laxative (e.g., bisacodyl) preferably 30 minutes after a meal 2
5. If constipation remains unresolved, consider switching to PEG or other agents 2

Special Populations

• Elderly patients: Use with caution due to increased risk of electrolyte disturbances 5
• Patients with heart failure: Use with caution due to risk of hypermagnesemia 5
• Children under 12: Dosing should be determined by a physician 1
• Pregnant women: Generally considered safe but should be used under medical supervision 5