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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.

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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.

How to take it

TimingEvening for a laxative effect; the Japanese constipation trials split 1.5 g across three doses during the day.
With food?With food and a full glass of water.
Worth knowingIf magnesium repletion rather than a laxative effect is the goal, oxide is the wrong buy — it is poorly absorbed, and citrate, malate or glycinate deliver far more of what is on the label.

Interactions

With medications

  • tetracycline and quinolone antibiotics

    magnesium chelates them and reduces antibiotic absorption separate by at least 2 hours

  • levothyroxine

    reduced levothyroxine absorption separate by at least 4 hours

  • bisphosphonates

    blocked absorption separate by at least 2 hours

  • pH-dependent drugs (atazanavir, ketoconazole, some iron salts)

    magnesium oxide raises gastric pH, which reduces absorption of drugs that need an acid stomach separate by at least 2 hours and ask your pharmacist about your specific medication

With other supplements

  • iron supplements

    the higher gastric pH plus mineral competition reduces iron absorption take iron at least 2 hours apart

Well-established interactions only — this is not a complete list. Always tell your doctor and pharmacist what you take.

Frequently asked questions

Is magnesium oxide poorly absorbed?

Yes. Magnesium oxide's fractional absorption is about 4%, and in a 60-day trial at 300 mg elemental magnesium a day it did not raise magnesium levels more than placebo. If you want to raise your magnesium status, citrate, malate or glycinate are better choices.

Does magnesium oxide work for constipation?

Yes, with moderate evidence. In a placebo-controlled trial in 33 women, 0.5 g magnesium oxide three times daily for 28 days improved chronic constipation in 70.6% versus 25.0% on placebo, and another trial found 1.5 g/day comparable to senna.

Does magnesium oxide help prevent migraines?

The evidence for magnesium oxide in migraine is limited: a few small single trials reported fewer headaches, with no consistent meta-analytic support. Its poor absorption also makes it an odd choice if magnesium is the active ingredient.

Is it safe to take magnesium oxide every day as a laxative?

Japan has issued safety warnings on high blood magnesium after case reports, including deaths, mostly in elderly people, long-term high-dose users and people with impaired kidney function. If you use magnesium oxide as a laxative long term, have your magnesium levels checked, and avoid it in advanced kidney disease.

Scientific references

Where to buy

Best Naturals, Magnesium Oxide, 500 mg , 180 Tablets

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