Magnesium Oxide: Uses, Dosage, and Safety Information

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

• National Institutes of Health, Office of Dietary Supplements. Magnesium: Fact Sheet for Health Professionals. Last updated June 2022. https://ods.od.nih.gov/factsheets/Magnesium-HealthProfessional/. Accessed March 2026.
• National Institutes of Health, Office of Dietary Supplements. Magnesium: Fact Sheet for Consumers. Last updated June 2022. https://ods.od.nih.gov/factsheets/Magnesium-Consumer/. Accessed March 2026.
• MedlinePlus. Magnesium in diet. Health topic. https://medlineplus.gov/ency/article/002423.htm. Accessed March 2026.