The cheapest and most concentrated magnesium salt, supplying about 60% elemental magnesium by weight but absorbed poorly — one controlled comparison put its bioavailability at around 4%. Most of it stays in the gut, where it acts as an osmotic laxative.
Magnesium oxide is the form found in most cheap supplements and the reason many people conclude magnesium "does not agree with them". Its poor absorption is not a marketing myth — it has been measured — and the unabsorbed fraction draws water into the bowel, which is why the same compound is sold as a laxative. That property gives it its one genuinely well-supported use: randomised trials, particularly from Japan, support magnesium oxide for chronic constipation. For raising magnesium status, or for anything downstream of that, better-absorbed salts make more sense.
Magnesia, MgO, Milk of magnesia (hydroxide relative)
Regulation: The Tolerable Upper Intake Level for supplemental magnesium is 350 mg/day for adults — and it applies only to supplements and medicines, not to magnesium in food, because the kidneys handle dietary magnesium without difficulty. Magnesium oxide is also sold as an over-the-counter laxative and antacid in many countries.
What the research used
The amounts, forms and durations published trials of Magnesium oxide actually administered.
These are the amounts published trials administered — not a recommendation, and not a dose to copy. Trial participants were screened, supervised and given a characterised preparation. What is in a retail bottle is frequently not what was studied, and the amount that suits one person is not general advice.
Studied for Recommended Dietary Allowance, adults
310–420 mg
Strong
310–320 mg/day for women and 400–420 mg/day for men, from all sources. Population surveys consistently show average intakes below this, which is the honest basis for the interest in magnesium supplements.
Studied for Tolerable Upper Intake Level, supplemental magnesium
350 mg
Strong
Set on diarrhoea, and applies only to magnesium from supplements and medicines. Food magnesium is not counted, which is unusual among upper limits and worth understanding.
Studied for Chronic constipation trials
500–1500 mg
Moderate
Magnesium oxide, daily · 4 weeks
Japanese randomised trials support this use, at doses well above the supplemental upper limit because the laxative effect is the intended outcome rather than an adverse one.
This is general information about food, not medical advice. It cannot diagnose, treat or replace guidance from a clinician who knows your history.
When it was taken
Each window says whether a trial compared it against an alternative, or whether it is convention and pharmacology. Most timing advice is the second kind.
None of the timings below were directly compared in a trial. They reflect how the research happened to dose, or what the pharmacology implies — which is not the same as a timing being better.
🌙Evening
Not enough
Magnesium is conventionally taken at night on the belief that it aids sleep. No trial has compared evening with morning dosing for any outcome — this is one of the most widely repeated untested timing claims in the supplement world.
Timing was not directly compared. This reflects convention or how the compound behaves in the body, not a trial showing this window works better.
🍽️With food
Not enough
Taken with food to reduce the laxative effect and nausea. Sensible for tolerability; not an absorption finding.
Timing was not directly compared. This reflects convention or how the compound behaves in the body, not a trial showing this window works better.
The evidence in depth
Best supported: Moderate
What each association actually rests on, and the papers behind it. A rating describes the state of the research, not how promising the supplement sounds. How we rate evidence.
Randomised trials support magnesium oxide for chronic constipation, where its poor absorption is the mechanism rather than a drawback. Effects appear within days and doses are higher than those used for nutritional purposes.
What may be going on: Unabsorbed magnesium salts are osmotically active, drawing water into the bowel lumen and softening stool.
Moderate evidence. Several human studies point the same way, with real caveats.
Magnesium has been studied for migraine, with trials most often using magnesium citrate or dicitrate at around 600 mg/day and reporting modest reductions in attack frequency. Magnesium oxide has been used in some paediatric trials with less convincing results, and its poor absorption is a plausible reason.
Limited evidence. Early human work, or mostly lab and animal research.
Meta-analysis · Neurological sciences : official journal of the Italian Neurological Society and of the Italian Society of Clinical Neurophysiology · 2025 · PMID 39404918
Evidence is limited for magnesium and sleep. The trials that exist are small, often in older adults with insomnia, use varied salts and doses, and rely on subjective sleep questionnaires with large placebo responses.
Not enough evidence. Not enough human research to say anything useful yet.
Deep research mode adds each paper’s own conclusion and stated limitations.
Things to know
Interactions, contraindications and dose limits, most serious first. Concentrated extracts interact with medication in ways the whole plant does not, so this section carries more weight here than it does on a food page.
Too much
Dangerous in kidney impairment
Magnesium is cleared by the kidneys, and in moderate to severe kidney disease supplements can cause hypermagnesaemia — nausea, low blood pressure, muscle weakness, confusion, and at high levels cardiac arrest. Fatal cases have followed magnesium-containing laxatives and antacids in people with reduced kidney function.
Interaction
Blocks absorption of several antibiotic classes and other drugs
Magnesium binds tetracyclines, fluoroquinolones and bisphosphonates in the gut, substantially reducing their absorption; separation by two to four hours is standard. It also interacts with levothyroxine and with gabapentin. Long-term proton pump inhibitor use lowers magnesium in the other direction.
Too much
Diarrhoea is the dose-limiting effect
The 350 mg/day upper limit for supplemental magnesium exists because of diarrhoea, and magnesium oxide reaches it faster than better-absorbed salts. Cramping and loose stools are the usual reason people abandon it.
Worth knowing
Elemental content versus salt weight
A "500 mg magnesium oxide" tablet supplies about 300 mg of elemental magnesium, of which very little is absorbed. Labels differ in which figure they state, which makes comparing magnesium products unusually confusing.
In the food catalogue
The whole-food form, where one exists. The evidence for a concentrated extract rarely transfers to the food, or the other way round — these are cross-links, not equivalents.