Onset of Action for 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
Magnesium hydroxide works through two primary mechanisms:
- Osmotic effect: The poorly absorbable magnesium ions exert an osmotic effect in the intestinal lumen, causing water retention that increases the fluidity of intestinal contents 2
- Local intestinal effect: This increased fluid content stimulates intestinal motility and facilitates bowel movements
Factors Affecting Onset Time
Several factors can influence how quickly milk of magnesia works:
- Dosage: Higher doses may work more rapidly
- Individual patient factors:
- Severity of constipation
- Gut transit time
- Hydration status
- Time of administration: Taking it on an empty stomach may accelerate action
- Concurrent medications: Some medications may interact with magnesium hydroxide
Clinical Applications
Milk of magnesia is commonly used for:
- Occasional constipation: As a short-term remedy 1
- Post-surgical bowel stimulation: Studies show it can be effective when started early after surgery 3
- Chronic idiopathic constipation (CIC): The American Gastroenterological Association suggests magnesium oxide as a treatment option for CIC 4, 5
Dosing Recommendations
- Standard dosing: 400-500 mg daily 5
- For constipation: The American Gastroenterological Association suggests starting at a lower dose, which may be increased if necessary 4, 5
Important Precautions
- Renal impairment: Avoid use in patients with renal insufficiency due to risk of hypermagnesemia 4, 5
- Monitoring: For prolonged use, monitor magnesium levels, especially in patients with impaired renal function 5, 6
- Potential complications: Hypermagnesemia can cause serious adverse effects including:
Clinical Pearl
When using milk of magnesia for constipation, adequate hydration should be encouraged to enhance its effectiveness and reduce the risk of adverse effects 4.