Magnesium oxide
Summary
Magnesium oxide is about 60% magnesium but was the least absorbed form in one comparison (about 4%). At 1.5 g/day it relieved chronic constipation better than placebo in two 4-week trials (grade B). At 1,000 mg/day it matched valproate for migraine frequency in one crossover trial (grade C). Older adults with kidney disease face a hypermagnesemia risk.
Key facts
- 1,000–1,500 mg/day of magnesium oxide (about 600–905 mg elemental magnesium), above the 350 mg supplement upper limit, in short trials[3][4][5][1]
- Adult RDA 310–420 mg/day magnesium from all sources; UL for supplemental magnesium 350 mg/day[1]
- Separate from bisphosphonates and tetracycline or quinolone antibiotics; long-term PPI use can lower magnesium[1]
- Diarrhea; magnesium oxide is one of the forms most often linked to it[1]
- 3 graded human outcomes on this page
- October 10, 2026
How it works
EstablishedBecause little magnesium oxide is absorbed, most of it stays in the gut, where magnesium salts draw water into the bowel by osmosis and soften stool. That is why it works as a laxative and why it is a poor choice when the goal is raising body magnesium. Absorbed magnesium acts as a cofactor in hundreds of enzyme reactions.[1][6]
What the human research says
Human trials only, one row per outcome. The form column shows what each trial actually tested. Grades follow our evidence-grading policy.
At a glance: strongest evidence per outcome
| Outcome | Grade | Effect | Strongest evidence | Rows |
|---|---|---|---|---|
| Constipation | Grade B | Improved | RCT · n = 90 · 2021 | 2 rows |
| Migraine | Grade C | Improved | Crossover RCT · n = 70 · 2021 | 1 row |
Every graded row
| Outcome | Effect | Grade | Best evidence | PMID |
|---|---|---|---|---|
| Overall improvement in chronic constipation | Improved | Grade B | RCT · n = 90 · 2021 Adults with chronic idiopathic constipation, 93% women Form: magnesium oxide · Dose: 1.5 g/day · Duration: 28 days 68.3% responded on magnesium oxide vs 11.7% on placebo (senna 69.2%). Spontaneous bowel movements and constipation quality of life improved; no severe treatment-related adverse events. | 32969946 (opens PubMed)Source [3] |
| Overall improvement in chronic constipation | Improved | Grade B | RCT · n = 34 · 2019 Japanese women with mild to moderate chronic constipation Form: magnesium oxide · Dose: 0.5 g three times daily (1.5 g/day) · Duration: 28 days 70.6% met the primary endpoint vs 25.0% on placebo; stool form, colonic transit time and quality of life improved. Complete spontaneous bowel movement response did not differ. | 31587548 (opens PubMed)Source [4] |
| Migraine attack frequency vs valproate | Improved | Grade C | Crossover RCT · n = 70 · 2021 Adults with migraine Form: magnesium oxide · Dose: 500 mg twice daily (1,000 mg/day) · Duration: 8 weeks per period (24-week study) Monthly attacks, headache days and duration were similar to valproate 800 mg/day, without significant adverse effects. | 30798472 (opens PubMed)Source [5] |
Forms
| Form | Elemental % | Absorption (human data) | GI tolerance | Studied for |
|---|---|---|---|---|
| Magnesium oxide | 60.3%[2] | Fractional absorption about 4% in one comparison, lower than magnesium chloride, lactate or aspartate.[6] | One of the forms most often linked to diarrhea.[1] | Constipation, migraine |
Compound weight → elemental magnesium
500 mg magnesium oxide ≈ 302 mg elemental magnesium
Studied doses
Doses studied in human trials ranged from 1,000 to 1,500 mg/day of magnesium oxide (about 600–905 mg elemental magnesium) for 4 to 8 weeks. That is above the 350 mg/day upper limit for supplemental magnesium, which these trials used under medical supervision.[3][4][5][1][2]
Intake reference: Adult RDA 310–420 mg/day from all sources (by age and sex). Upper limit for supplemental magnesium: 350 mg/day for adults.[1] The upper limit covers supplements and medicines only, not magnesium in food. NIH ODS fact sheet (updated January 6, 2026).
Dose math: how much compound supplies that much magnesium
| Form (elemental %) | 350 mg magnesium350 mg/day: adult upper limit from supplements (a ceiling, not a target) | 603 mg magnesiumAbout 603 mg/day: elemental magnesium in the 1,000 mg/day migraine trial dose | 905 mg magnesiumAbout 905 mg/day: elemental magnesium in the 1.5 g/day constipation trial dose |
|---|---|---|---|
| Magnesium oxide (60.3%) | ≈ 580 mg compound | ≈ 1,000 mg compound | ≈ 1,500 mg compound |
Interactions
Published interactions only: each row cites a source. No row means none was found in our sources, not that a combination is safe.
Medicines
Medicines that can lower levels
- Proton pump inhibitors (long-term)NIH fact sheetUse for more than a year can cause low magnesium, which supplements may not fully correct.[1]
Who should be careful
- Kidney disease and older age
- In a case series, four people over 65 with kidney impairment developed severe hypermagnesemia from magnesium oxide laxatives, one fatally. Authors advise checking blood magnesium in high-risk patients.[7]
- Raising magnesium status
- Magnesium oxide was the least absorbed form in one comparison, so it is a poor fit when the aim is to correct low magnesium.[6]
- Doses above the upper limit
- The constipation and migraine trials used more than the 350 mg/day supplement upper limit; doses that high are used under clinician supervision.[1][3][5]
- Medicines
- Antibiotic and bisphosphonate timing matters; see interactions.[1]
Talk to your pharmacist or clinician before starting a supplement, especially if you take prescription medicines.
How to read the label
- Label dose vs clinical doseSupplement Facts list elemental magnesium: a 400 mg magnesium serving from oxide is about 663 mg of the compound. Constipation trials used 1.5 g of the compound (about 905 mg magnesium), well above the 350 mg supplement upper limit.[1][2][3]
- Read the elemental numberFDA labels show the weight of magnesium itself in Supplement Facts. A front label saying "500 mg magnesium oxide" means about 300 mg of magnesium at 60.3%.[1][2]
- Count every source against the upper limitThe 350 mg/day UL counts magnesium from all supplements and medicines combined, including antacids and laxatives that contain magnesium oxide.[1]
Cost per studied dose, without the sales pitch
Compare any product you already have by the amount it delivers, not by capsule count or front-of-pack weight.
Cost per studied dose
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Frequently asked questions
Is magnesium oxide poorly absorbed?
Yes, relative to other forms. One comparison found about 4% fractional absorption, versus higher and similar values for magnesium chloride, lactate and aspartate.[6]
Does magnesium oxide help constipation?
Two 4-week placebo-controlled trials at 1.5 g/day found about 68–71% of people improved, versus 12–25% on placebo.[3][4]
How much magnesium is in magnesium oxide?
About 60.3% by weight, so 500 mg of magnesium oxide contains roughly 300 mg of magnesium.[2]
Is magnesium oxide good for migraine?
One crossover trial found 1,000 mg/day performed similarly to valproate for attack frequency. It had no placebo arm, so the evidence is limited.[5]
Who should be careful with magnesium oxide?
Older adults and people with kidney disease: case reports describe severe, sometimes fatal, hypermagnesemia from magnesium oxide laxatives in this group.[7]
Sources
- [1]Magnesium: Fact Sheet for Health Professionals (updated January 6, 2026). NIH Office of Dietary Supplements, 2026. fact-sheet
- [2]Magnesium oxide (CID 14792): MgO, MW 40.305. PubChem, NCBI, 2026. database
- [3]Senna Versus Magnesium Oxide for the Treatment of Chronic Constipation: A Randomized, Placebo-Controlled Trial.. Am J Gastroenterol, 2021. RCT
- [4]A Randomized Double-blind Placebo-controlled Trial on the Effect of Magnesium Oxide in Patients With Chronic Constipation.. J Neurogastroenterol Motil, 2019. RCT
- [5]The efficacy of magnesium oxide and sodium valproate in prevention of migraine headache: a randomized, controlled, double-blind, crossover study.. Acta Neurol Belg, 2021. crossover-RCT
- [6]Bioavailability of US commercial magnesium preparations.. Magnes Res, 2001. PK-study
- [7]Severe hypermagnesemia induced by magnesium oxide ingestion: a case series.. CEN Case Rep, 2019. observational
What changed
- First draft. Every PMID checked against NCBI E-utilities esummary (title and year taken from the record) and abstracts read for the cited rows.
- Adversarial review pass: every claim re-checked against the cited abstract (or full text where noted), PMIDs re-verified via NCBI esummary, unsupported label-market statements removed, two evidence rows with unreported pooled n removed. Page set indexable.