What is the onset of action of milk of magnesia (magnesium hydroxide)?

Onset of Action of Milk of Magnesia (Magnesium Hydroxide)

Milk of magnesia (magnesium hydroxide) typically produces a bowel movement within 30 minutes to 6 hours after administration. 1

Mechanism of Action

Milk of magnesia works as an osmotic laxative through the following mechanisms:

• Draws water into the intestinal lumen, increasing stool water content
• Distends the colon, stimulating peristalsis
• Creates softer stool that is easier to pass

Clinical Application

The American Gastroenterological Association recommends milk of magnesia as an inexpensive osmotic agent for the management of chronic constipation 2. The typical dosage is 1 oz (30 mL) twice daily, with an approximate daily cost of less than $1.

Treatment Algorithm for Constipation

1. First-line therapy: Gradual increase in fiber intake (both dietary and supplements)
2. Second-line therapy: Osmotic laxatives (milk of magnesia or polyethylene glycol)
3. Third-line therapy: Add stimulant laxatives (e.g., bisacodyl or glycerol suppositories) if needed
   • Stimulant laxatives are preferably administered 30 minutes after meals to synergize with the gastrocolonic response
4. Fourth-line therapy: Consider newer agents (lubiprostone, linaclotide) if symptoms don't respond to conventional laxatives

Comparative Efficacy

When compared to other laxatives:

• Meta-analysis of 3 studies with 211 participants showed that polyethylene glycol (PEG) produced slightly more stools per week than milk of magnesia (mean difference 0.69,95% CI 0.48 to 0.89), though this difference may not be clinically significant 3
• One study found milk of magnesia to be more effective than lactulose in producing bowel movements (mean difference -1.51 stools per week, 95% CI -2.63 to -0.39) 3

Safety Considerations

Potential Adverse Effects

• Common: Abdominal discomfort, cramping, diarrhea if overdosed
• Rare but serious: Hypermagnesemia, particularly in patients with:
  • Renal insufficiency
  • Elderly patients
  • Patients taking other magnesium-containing medications

Risk of Hypermagnesemia

Hypermagnesemia can occur with excessive use of magnesium-containing products, especially in patients with impaired renal function 4. Clinical manifestations depend on serum magnesium levels:

• Mild (2.2-2.5 mEq/L): Often asymptomatic
• Moderate (2.5-5.0 mEq/L): Hyporeflexia, nausea, vomiting, facial flushing, lethargy
• Severe (>5.0 mEq/L): Complete loss of deep tendon reflexes, severe hypotension, respiratory depression, heart blocks, coma

Case reports have documented paralytic ileus 5 and even fatal hypermagnesemia 6 with excessive magnesium intake, highlighting the importance of appropriate dosing and patient selection.

Practical Considerations

• Milk of magnesia should be taken with a full glass of water
• For best results, it should be taken on an empty stomach
• Patients should be advised that the onset of action is typically within 30 minutes to 6 hours
• Regular use should be avoided without medical supervision, as it may lead to electrolyte imbalances or dependence

Milk of magnesia remains a cost-effective option for managing constipation when used appropriately, with a predictable onset of action within 30 minutes to 6 hours after administration.