Supplement analysis backed by scientific evidence · PT

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Moderate evidenceVitamins & minerals

Magnesium Oxide

The cheapest magnesium and the worst absorbed — around 4% fractional absorption, and no better than placebo at raising magnesium status in a 60-day trial. It is genuinely good at one thing: it works as a laxative for chronic constipation, with proper randomised evidence behind it.

What the science says

  • Poor bioavailability as a repletion source Moderate evidence

    A urinary-excretion study of four US commercial preparations measured magnesium oxide's fractional absorption at about 4%, versus significantly higher and equivalent absorption from magnesium chloride, lactate and aspartate. In a 60-day randomised double-blind trial at 300 mg elemental Mg/day, oxide produced no change in serum, salivary or urinary magnesium versus placebo, while citrate did.

  • Works for chronic constipation Moderate evidence

    In a randomised double-blind placebo-controlled trial of 33 women with mild-to-moderate chronic constipation, 0.5 g magnesium oxide three times daily for 28 days met the primary improvement endpoint in 70.6% versus 25.0% on placebo, and significantly shortened colonic transit time. A separate randomised placebo-controlled trial found 1.5 g/day magnesium oxide comparable to senna for chronic constipation.

  • Migraine prophylaxis evidence is weak and mixed Limited evidence

    A randomised double-blind trial in children used 9 mg/kg/day magnesium oxide for 16 weeks, and a crossover trial compared 500 mg/day against sodium valproate, both reporting reductions in headache frequency. These are small single trials with no consistent meta-analytic support, and the poor absorption of oxide makes it an odd choice if magnesium is the active principle.

  • Best value per milligram of elemental magnesium — with a catch Moderate evidence

    Magnesium oxide is about 60% elemental magnesium by weight, the highest of any common form, which is why it dominates cheap multivitamins and big-count bottles. The catch is that the milligrams on the label are not the milligrams you absorb.

Dosage & safety

Studied doseFor general supplementation, 400-500 mg magnesium oxide provides roughly 240-300 mg elemental magnesium on paper — expect a fraction of that to be absorbed. For chronic constipation, Japanese trials used 1.5 g/day magnesium oxide (0.5 g three times daily) for 4 weeks. Adult RDA for total magnesium intake is 310-420 mg/day.
SafetyDiarrhoea and abdominal cramping are common and are the main dose-limiting effect. Japan, where magnesium oxide is a mainstream prescription laxative, has issued regulatory safety communications on hypermagnesemia after case reports including fatalities, mostly in elderly patients, those on long-term high doses and those with impaired renal function — check magnesium levels if you use laxative doses chronically. Avoid in advanced chronic kidney disease. Separate by 2 hours from tetracyclines, quinolones, bisphosphonates and levothyroxine; magnesium oxide also raises gastric pH and can alter absorption of pH-dependent drugs.

Scientific references

Where to buy

Best Naturals, Magnesium Oxide, 500 mg , 180 Tablets

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