Mechanism of Action of Milk of Magnesia
Milk of magnesia (magnesium hydroxide) works primarily as an osmotic laxative by drawing water into the intestinal lumen through poorly absorbable magnesium ions, which increases stool fluidity and produces a bowel movement typically within 30 minutes to 6 hours. 1
Primary Osmotic Mechanism
- The magnesium ions exert an osmotic effect in the intestinal lumen, causing water retention that increases the fluidity of intestinal contents and results in laxation. 2
- The poorly absorbable nature of magnesium salts is essential to this mechanism—the ions remain in the gut rather than being absorbed systemically, maintaining the osmotic gradient. 2
- This osmotic action is the same mechanism shared by other magnesium salts (magnesium sulfate, magnesium oxide), though magnesium hydroxide also has antacid properties that neutralize stomach acid. 2
Additional Mechanisms
- Beyond the local osmotic effect, magnesium may stimulate release of digestive polypeptides such as cholecystokinin and activate constitutive nitric oxide synthase, which could contribute to the laxative action. 2
- These hormonal and enzymatic effects represent intrinsic mechanisms that complement the primary osmotic action. 2
Clinical Context and Dosing
- The American Gastroenterological Association recommends milk of magnesia as an inexpensive first-line osmotic agent for chronic constipation, typically dosed at 1 oz (30 mL) twice daily. 3
- Magnesium salts should be used cautiously in patients with renal impairment, as excessive doses can lead to hypermagnesemia due to systemic absorption. 3
- The European Society for Medical Oncology notes that while magnesium salts are commonly used osmotic laxatives, they require adequate fluid intake and monitoring in patients with compromised renal function. 3
Safety Considerations
- Under normal circumstances, pharmacological doses of magnesium hydroxide are safe for short-term use, though prolonged use or high doses may allow sufficient systemic absorption to cause renal or other organ toxicity. 2
- One study comparing milk of magnesia to polyethylene glycol found no clinically significant side effects or blood abnormalities in children treated for 12 months, demonstrating good long-term safety. 4
- Common adverse effects include abdominal discomfort, flatulence, and diarrhea if dosed too aggressively. 5