What is the typical dosage and use of milk of magnesia for constipation or as an antacid?

Milk of Magnesia Dosage and Use for Constipation and as an Antacid

Milk of magnesia (magnesium hydroxide) is recommended as an effective, low-cost osmotic laxative for constipation at a dose of 30-60 mL (1-2 oz) daily or twice daily, and as an antacid at a dose of 5-15 mL as needed for symptom relief. 1

Mechanism of Action

Milk of magnesia works through two primary mechanisms:

• As a laxative: Poorly absorbed magnesium ions create an osmotic gradient in the intestinal lumen, drawing water into the bowel, increasing stool fluidity, and stimulating peristalsis 2
• As an antacid: Magnesium hydroxide neutralizes stomach acid, providing relief from heartburn and indigestion

Dosage Guidelines

For Constipation:

• Adults: 30-60 mL (1-2 oz) daily, preferably at bedtime
• Children: Dosage should be adjusted by age and weight (consult product labeling)
• Onset of action: 6-8 hours (typically works overnight when taken at bedtime)

As an Antacid:

• Adults: 5-15 mL as needed for symptom relief
• Maximum: 60 mL in 24 hours
• Onset of action: Within minutes

Clinical Evidence and Recommendations

The American Gastroenterological Association recommends milk of magnesia as an inexpensive osmotic laxative for constipation management 1. Clinical evidence shows:

• Magnesium oxide (similar to milk of magnesia) may increase complete spontaneous bowel movements by 4.29 per week compared to placebo 1
• Magnesium-based laxatives demonstrate better stool consistency based on the Bristol Stool Form Scale 1
• In comparative studies, polyethylene glycol showed slightly better efficacy than milk of magnesia, but the difference may not be clinically significant (0.69 more stools per week) 3

Precautions and Contraindications

• Renal impairment: Avoid in patients with creatinine clearance <20 mL/min due to risk of hypermagnesemia 1, 4
• Bowel obstruction: Contraindicated in suspected bowel obstruction 5
• Prolonged use: May lead to electrolyte disturbances and magnesium toxicity, particularly in elderly patients or those with impaired renal function 4

Adverse Effects

• Common: Abdominal cramping, diarrhea, nausea
• Serious but rare: Hypermagnesemia (especially in renal impairment), presenting as:
  • Hypotension
  • Respiratory depression
  • Cardiac arrhythmias
  • Mental status changes 5, 4

Clinical Algorithm for Use

1. For occasional constipation:

   • Start with 30 mL (1 oz) at bedtime
   • May increase to 60 mL (2 oz) if needed
   • Expect results within 6-8 hours

2. For chronic constipation:

   • Consider as part of a stepwise approach: a. Start with lifestyle modifications (increased fluid, fiber, physical activity) b. Add milk of magnesia 30-60 mL daily c. If inadequate response, add or switch to a stimulant laxative (e.g., bisacodyl, senna) d. For refractory cases, consider prescription medications or specialist referral 1

3. For antacid use:

   • Take 5-15 mL as needed for symptoms
   • Do not exceed 60 mL in 24 hours
   • Consider H2 blockers or PPIs for frequent symptoms requiring regular antacid use

Important Clinical Pearls

• Efficacy: Milk of magnesia is generally as effective as other osmotic laxatives but may be less well-tolerated than polyethylene glycol 3, 6
• Cost-effectiveness: Significantly less expensive than newer agents ($1/day vs. $7-9/day for newer prescription laxatives) 1
• Hydration: Ensure adequate fluid intake when using milk of magnesia to prevent dehydration
• Monitoring: For long-term use, periodic monitoring of serum magnesium and renal function is advisable, especially in elderly patients 4
• Palatability: The chalky taste may reduce compliance compared to other laxatives; compliance rates of 65% have been reported (vs. 95% for PEG) 6

Milk of magnesia remains a valuable first-line option for constipation management due to its efficacy, safety profile, over-the-counter availability, and low cost when used appropriately.