Why is magnesium oxide so commonly used if absorption is poor?

Magnesium oxide is significantly cheaper than chelated forms and has high elemental magnesium by weight. Many low-cost supplements use it to display high magnesium numbers on the label while delivering little to the body.

What is the absorbed dose difference between glycinate and oxide?

At the same elemental magnesium dose, glycinate and oxide do not deliver the same amount of absorbed magnesium. Oxide is the weaker choice for raising serum magnesium at standard doses, which is why it is more often used for its laxative effect.

Can magnesium glycinate cause diarrhea?

At normal doses (200–400 mg elemental/day), magnesium glycinate is well-tolerated. Unlike oxide, it does not have significant osmotic laxative activity. Very high doses may cause loose stools in some individuals.

Magnesium Glycinate vs Oxide: Absorption and Tolerability

Magnesium glycinate is absorbed more efficiently than magnesium oxide, a difference that matters when the goal is systemic magnesium support rather than a laxative effect.

Updated 2026 · Reviewed by Dr. Brennan Commerford, D.C.

Our Recommendation

Magnesium Glycinate

Magnesium glycinate is better absorbed and better tolerated than magnesium oxide — the most common cheap filler form.

Magnesium Glycinate vs Magnesium Oxide

FormulaForge Choice

Magnesium Glycinate

Pros

+Among the better-absorbed magnesium forms
+Gentle on the GI tract — does not cause the laxative effect of oxide
+Glycine amino acid co-factor may support relaxation and sleep quality

Cons

-Higher cost per milligram of elemental magnesium than oxide
-Lower elemental magnesium percentage by weight than oxide

Best For

Anyone seeking to effectively support serum magnesium levels for muscle function, sleep, and stress resilience.

S1 bioavailability tier — amino-acid chelated form

Avoid

Magnesium Oxide

Pros

+Very low cost per gram
+High elemental magnesium percentage by weight (60%)
+Effective as an osmotic laxative

Cons

-Poorly absorbed relative to chelated forms
-Causes significant GI distress and osmotic diarrhea at magnesium-relevant doses
-Poorly suited for systemic magnesium repletion

Best For

Not recommended for systemic magnesium support. Appropriate only as a laxative, not a magnesium supplement.

S4 bioavailability tier — inorganic oxide salt

Frequently Asked Questions

Why is magnesium oxide so commonly used if absorption is poor?: Magnesium oxide is significantly cheaper than chelated forms and has high elemental magnesium by weight. Many low-cost supplements use it to display high magnesium numbers on the label while delivering little to the body.
What is the absorbed dose difference between glycinate and oxide?: At the same elemental magnesium dose, glycinate and oxide do not deliver the same amount of absorbed magnesium. Oxide is the weaker choice for raising serum magnesium at standard doses, which is why it is more often used for its laxative effect.
Can magnesium glycinate cause diarrhea?: At normal doses (200–400 mg elemental/day), magnesium glycinate is well-tolerated. Unlike oxide, it does not have significant osmotic laxative activity. Very high doses may cause loose stools in some individuals.

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Not All Magnesium Forms Absorb the Same — Here's Why

Most people buy magnesium oxide. Little of it reaches your bloodstream; the rest acts as an osmotic laxative. Our magnesium guide ranks all 8 forms by actual bioavailability and which goal each serves best.

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FormulaForge formulates and sells supplements containing the ingredients discussed on this page. Our formulary recommendations are based on peer-reviewed bioavailability research. All cited studies are independently verifiable.