Milk of Magnesia Does Not Cause Dehydration—It Actually Draws Water INTO the Bowel
Milk of magnesia (magnesium hydroxide) does not dehydrate you; rather, it works by creating an osmotic gradient that pulls water into the intestinal lumen, which is its therapeutic mechanism for treating constipation. 1
Mechanism of Action
Magnesium hydroxide functions as an osmotic laxative by remaining poorly absorbed in the gastrointestinal tract, where it creates an osmotic gradient that leads to net secretion of water and electrolytes into the bowel lumen 1, 2
This water retention in the intestines increases stool fluidity and promotes bowel movements—the opposite of systemic dehydration 1, 2
Important Clinical Caveats
When Dehydration CAN Occur (Paradoxical Effect)
The only scenario where milk of magnesia might contribute to dehydration is through excessive dosing that causes severe diarrhea, leading to fluid losses that exceed intake. 3
A case report documented paradoxical hypomagnesemia and dehydration in a patient who overdosed on magnesium hydroxide, developing massive watery diarrhea with subsequent electrolyte losses 3
An infant case showed hypertonic dehydration with metabolic alkalosis after magnesium oxide administration in the setting of diarrhea 4, 5
Key Safety Considerations
Avoid in patients with renal insufficiency (creatinine clearance <20 mg/dL) due to risk of hypermagnesemia, as systemic magnesium regulation depends on renal excretion 1
Severe hypermagnesemia from excessive use in renal failure can cause life-threatening bradycardia, asystole, shock, and respiratory failure 6
The FDA label warns to stop use if no bowel movement occurs after use, as this could indicate a serious condition requiring medical evaluation 7
Clinical Bottom Line
Under normal therapeutic dosing, milk of magnesia does not cause dehydration—it hydrates the stool. The 2023 AGA-ACG guidelines support magnesium oxide as an attractive first-line option for chronic constipation due to its efficacy, tolerability, over-the-counter availability, and low cost 1. Dehydration only becomes a concern with overdosing that causes excessive diarrhea or in patients with significant renal impairment who develop hypermagnesemia.