Magnesium glycinate wins decisively. Magnesium oxide has approximately 4% bioavailability — meaning 96% of the dose passes through unabsorbed and causes diarrhoea. Glycinate is absorbed at roughly 80% relative bioavailability with zero laxative effect. The higher price of glycinate is justified by its vastly superior efficacy.
Overview
Magnesium oxide is the most common form found in cheap multivitamins and budget supplements — and it's also the worst-absorbed form available. This comparison explains why paying more for glycinate almost always makes more sense.
What Is Magnesium Glycinate?
Magnesium chelated to glycine — one of the most bioavailable forms available, with excellent GI tolerance and added neurological benefits from the glycine component.
Best for: Any magnesium deficiency goal, Sleep, Anxiety, Muscle health.
Standard dose: 200–400 mg elemental magnesium/day.
Side effects: Minimal — no laxative effect.
What Is Magnesium Oxide?
The most common, cheapest form of supplemental magnesium. A simple inorganic salt with very poor solubility in the gut. Used primarily as an antacid or laxative, not as a nutritional magnesium supplement.
Best for: Laxative effect (this is its main legitimate use), Antacid (heartburn), Filler in cheap multivitamins (not recommended).
Standard dose: Not recommended as a nutritional supplement — the dose that avoids diarrhoea is too low to be therapeutically meaningful.
Side effects: Diarrhoea, GI cramping, bloating at almost any meaningful dose. The laxative effect is the point if used as a laxative..
Evidence & Absorption Scores
We scored both on four dimensions: quality of clinical evidence, bioavailability, GI tolerance, and value for money. Scores are out of 10:
Head-to-Head Comparison
| Category | ▲ Magnesium Glycinate | ▲ Magnesium Oxide |
|---|---|---|
| Bioavailability | ~80% — excellent | ~4% — negligible |
| GI Tolerance | Excellent — no laxative effect | Poor — causes diarrhoea at effective doses |
| Effective Dose Achievable | Yes — 200–400 mg elemental Mg easily tolerated | No — effective doses universally cause loose stools |
| Sleep Benefit | Excellent | Minimal (insufficient absorption) |
| Anxiety Benefit | Excellent | Minimal |
| Cost per Effective Dose | Moderate — but affordable per absorbed mg | Appears cheap; worst value per absorbed mg |
| Laxative Use | Not effective | Effective — this is its primary role |
| Found In | Quality supplements, standalone | Cheap multivitamins, antacids |
Best Uses for Each
✅ Magnesium Glycinate — Best For
- Any magnesium deficiency goal
- Sleep
- Anxiety
- Muscle health
✅ Magnesium Oxide — Best For
- Laxative effect (this is its main legitimate use)
- Antacid (heartburn)
- Filler in cheap multivitamins (not recommended)
Who Should Choose Magnesium Glycinate?
▲ Choose Magnesium if:
Almost everyone supplementing for magnesium health benefits. Glycinate is the standard recommendation from nutrition scientists, dietitians, and functional medicine practitioners.
▲ Choose Magnesium if:
Only if you specifically need a laxative effect (and even then, magnesium citrate is more pleasant). Never choose magnesium oxide as your primary magnesium supplement.
Can You Take Both?
No reason to combine these. If you own a magnesium oxide supplement, you would be better served switching to glycinate, malate, or citrate.
Frequently Asked Questions
The Bottom Line
Magnesium glycinate wins decisively. Magnesium oxide has approximately 4% bioavailability — meaning 96% of the dose passes through unabsorbed and causes diarrhoea. Glycinate is absorbed at roughly 80% relative bioavailability with zero laxative effect. The higher price of glycinate is justified by its vastly superior efficacy.