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Gut Health

Magnesium Oxide

Evidence Level
Moderate
2 Clinical Trials
5 Documented Benefits
3/5 Evidence Score

Magnesium oxide is magnesium bound to oxygen, about 60 percent magnesium by weight, and the cheapest and one of the most common forms in supplements and multivitamins. It is poorly soluble, so absorption is less reliable than with citrate: one urine-based study estimated 4 percent absorption, while a small tracer study found it absorbed about as well as glycinate. In several placebo-controlled trials daily oxide raised blood magnesium over weeks, though others found no significant rise. The unabsorbed portion draws water into the bowel, which is why it loosens stools and is used as an antacid and, in Japan, as a laxative.

Studied Dose 300-500 mg/day of magnesium as oxide in supplement trials; about 0.9 g/day of magnesium in laxative trials.
Active Compound Magnesium oxide.

Benefits

Antacid Use

Magnesium oxide neutralizes stomach acid and is a listed active ingredient in FDA's over-the-counter antacid monograph, where labels warn it may have a laxative effect.

Supported by Trial 1, Trial 2

Constipation/Laxative

Unabsorbed magnesium draws water into the bowel. In two small Japanese placebo-controlled trials at laxative doses far above the supplement limit, about 70 percent improved on magnesium oxide against 11 to 25 percent on placebo. It is not a listed ingredient in FDA's laxative monograph.

Supported by Trial 2

High Elemental Magnesium Content

About 60 percent magnesium by weight, so a small tablet carries a lot of magnesium. Absorption is less reliable than with citrate, and absorption estimates vary widely by study and method, so the label number does not tell you how much is absorbed.

Supported by Trial 1, Trial 2

Lower Cost

The cheapest form we priced, at about $0.03 per 250 mg of magnesium for store-brand tablets, against about $0.20 to $0.70 for glycinate.

Supported by Trial 1, Trial 2

Migraine Adjunct (Some Evidence)

Some migraine trials used magnesium oxide; the best-known positive adult trial used a citrate form, and a pediatric oxide trial found no significant difference from placebo in headache frequency. Migraine doses exceed the supplement limit and need clinician supervision.

Supported by Trial 1, Trial 2

Mechanism of action

1

Low Solubility

Magnesium oxide is nearly insoluble in water and only partly dissolves in stomach acid (43 percent under simulated peak acid in one lab test), so absorption depends on stomach acid and is less reliable than with soluble salts such as citrate.

2

Antacid Reaction

MgO + 2HCl gives MgCl2 + H2O, neutralizing stomach acid.

3

Osmotic Laxative Effect

Unabsorbed magnesium in the bowel pulls water into the gut by osmosis, softening stool and increasing its volume.

Clinical trials

1
Magnesium Oxide vs Citrate Bioavailability

Randomized, double-blind, placebo-controlled trial, 60 days. (Magnes Res, 2003)

46 healthy adults taking 300 mg of magnesium a day.

Citrate and an amino acid chelate raised urine magnesium more than oxide, citrate gave the highest blood levels, and oxide showed no difference from placebo. A later 24-week trial in 164 adults found similar plasma magnesium effects with oxide, citrate and sulfate.

2
Magnesium Oxide vs Senna vs Placebo for Constipation
PubMed

Randomized, placebo-controlled trial. (Am J Gastroenterol, 2021)

90 adults with chronic constipation in Japan.

Overall response was 68.3 percent with magnesium oxide, 69.2 percent with senna and 11.7 percent with placebo, at laxative doses well above the supplement limit.

Side effects and drug interactions

Common Potential side effects

Loose stools or diarrhea; NIH lists oxide among the forms most often reported to cause diarrhea.
Digestive changes were reported by half the magnesium group in one 400 mg trial, against 7 percent on placebo.
Bloating, gas and cramping.
High blood magnesium, mostly reported in older adults using it long term as a laxative and in people with kidney disease.

Important Drug interactions

Bisphosphonates: take magnesium at least 2 hours apart.
Tetracycline and quinolone antibiotics: take them 2 hours before or 4 to 6 hours after magnesium.
Acid-reducing drugs (PPIs, H2 blockers) may reduce how well magnesium oxide dissolves.
Levothyroxine: separate doses, as magnesium may reduce its absorption.
Diuretics can change magnesium balance; ask your prescriber.

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Frequently asked questions about Magnesium Oxide

What is magnesium oxide, and is it well absorbed?

Magnesium oxide is the cheapest common magnesium form and about 60 percent magnesium by weight. It is absorbed less reliably than citrate, but the often-quoted 4 percent figure comes from one urine-based study, and several trials found daily oxide raised blood magnesium over weeks.

What is magnesium oxide best used for?

It is an inexpensive way to get magnesium if it agrees with you, and it is used as an antacid. Its laxative effect means it often loosens stools; people who want the most dependable rise in magnesium, or who get loose stools, may prefer citrate or glycinate.

How much magnesium oxide is safe to take?

Read the label for elemental magnesium: a 400 mg magnesium oxide tablet supplies about 241 mg of magnesium. Keep supplemental magnesium from all sources at or under 350 mg a day unless a clinician advises otherwise, and ask first if you have kidney disease.

Does magnesium oxide cause diarrhea?

It can, because unabsorbed magnesium pulls water into the intestines. Lowering the dose or switching to another form, such as glycinate, are common options.

What is Magnesium Oxide?

Magnesium oxide is magnesium bound to oxygen, about 60 percent magnesium by weight, and the cheapest and one of the most common forms in supplements and multivitamins.

What is Magnesium Oxide used for?

Magnesium Oxide is researched primarily for Gut Health. Magnesium oxide neutralizes stomach acid and is a listed active ingredient in FDA's over-the-counter antacid monograph, where labels warn it may have a laxative effect.

Is Magnesium Oxide safe, and does it have side effects?

Side effects and cautions reported for Magnesium Oxide include: Loose stools or diarrhea; NIH lists oxide among the forms most often reported to cause diarrhea. Digestive changes were reported by half the magnesium group in one 400 mg trial, against 7 percent on placebo. It may also interact with some medications. Magnesium Oxide is not right for everyone, so check with a healthcare provider first if you are pregnant or breastfeeding, have a medical condition, or take prescription medication.

Does Magnesium Oxide interact with any medications?

Possible interactions include: Bisphosphonates: take magnesium at least 2 hours apart. Tetracycline and quinolone antibiotics: take them 2 hours before or 4 to 6 hours after magnesium. If you take prescription medication, check with a pharmacist or doctor before using it.

How strong is the scientific evidence for Magnesium Oxide?

NutraSmarts rates the evidence for Magnesium Oxide as Moderate (3 out of 5). It is backed by 2 study summaries and 1 cited reference summarized on this page. A higher rating reflects more, larger, and better-designed human studies.

References(1 citations)

Evidence ratings on NutraSmarts are based on the totality of human clinical research, with emphasis on randomized controlled trials, meta-analyses, and systematic reviews. The references below directly support claims made throughout this page.

  1. Zhang X, Li Y, Del Gobbo LC, Rosanoff A, Wang J. Effects of Magnesium Supplementation on Blood Pressure: A Meta-Analysis of Randomized Double-Blind Placebo-Controlled Trials. Hypertension. 2016;68(2):324-33..PubMedUsed to support: Meta-analysis of magnesium supplementation on blood pressure. This meta-analysis pooled magnesium supplementation broadly across various magnesium forms, not magnesium oxide specifically; because oxide is only ~4% absorbed, it is a poor form for systemic magnesium repletion, so this blood-pressure finding should not be read as oxide-specific.