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Magnesium Oxide: Uses, Dosage, and Safety Information

Last reviewed: March 2026

Overview

Magnesium oxide is a mineral supplement containing approximately 60% elemental magnesium by weight—the highest concentration among common magnesium forms. It is widely used to address magnesium deficiency and as an over-the-counter laxative for occasional constipation.

Despite its high elemental magnesium content, magnesium oxide has relatively low bioavailability (approximately 4%), meaning only a small percentage is absorbed into the bloodstream. This makes it less ideal for correcting deficiency but effective as an osmotic laxative.

Common Brand Names

  • Mag-Ox 400
  • Uro-Mag
  • Phillips’ Milk of Magnesia (contains magnesium hydroxide, similar properties)

Primary Uses

Magnesium oxide is commonly used for:

  • Magnesium supplementation — addressing dietary insufficiency
  • Occasional constipation — works as an osmotic laxative
  • Heartburn and indigestion — antacid properties neutralize stomach acid
  • Migraine prevention — some evidence supports use for reducing migraine frequency

How It Works

Magnesium oxide works through several mechanisms depending on the use:

  • As a supplement: Provides elemental magnesium for cellular processes including muscle function, nerve transmission, and energy production
  • As a laxative: Draws water into the intestines through osmosis, softening stool and stimulating bowel movements
  • As an antacid: Neutralizes hydrochloric acid in the stomach

Safety Considerations

Magnesium oxide is generally safe when used as directed. Key considerations include:

  • Kidney function: People with kidney disease should consult a healthcare provider, as magnesium is excreted by the kidneys
  • Medication interactions: Can interfere with absorption of certain antibiotics (tetracyclines, fluoroquinolones) and bisphosphonates
  • Timing: Take 2 hours apart from other medications
  • GI tolerance: More likely to cause digestive side effects than other magnesium forms

Sources