Yes, magnesium hydroxide is the same as milk of magnesia
Milk of magnesia is simply the brand/common name for a liquid suspension of magnesium hydroxide. 1
What They Are
- Magnesium hydroxide is the chemical compound (Mg(OH)₂)
- Milk of magnesia is the white liquid suspension form containing magnesium hydroxide as the active ingredient
- FDA labeling confirms that milk of magnesia contains magnesium hydroxide as its sole active ingredient at 400 mg per 5 mL 1 or 2400 mg per 30 mL 1
Clinical Use
Both terms refer to the same medication used as:
- Antacid for heartburn/indigestion
- Laxative for constipation 1
The 2023 AGA-ACG guidelines discuss magnesium for chronic constipation, though they specifically evaluated magnesium oxide (MgO) rather than magnesium hydroxide in clinical trials 2. However, the mechanism is identical—creating an osmotic gradient in the GI tract that draws water into the lumen 2.
Important Clinical Distinctions
While the terms are interchangeable, note that:
- Different magnesium formulations exist (oxide, citrate, hydroxide, sulfate) with varying bioavailability 2
- Magnesium hydroxide has approximately 15% bioavailability when taken orally 3
- The liquid "milk of magnesia" form is specifically the hydroxide suspension, not other magnesium salts
Critical Safety Caveat
Avoid in patients with creatinine clearance <20 mL/dL due to risk of life-threatening hypermagnesemia, as renal excretion maintains magnesium homeostasis 2, 4. One case report documented near-fatal hypermagnesemia with cardiac arrest in a patient with end-stage renal disease using milk of magnesia 4.
Paradoxically, excessive ingestion can also cause hypomagnesemia through severe diarrhea-induced losses 5.