SupplementsMineralsMagnesium
Magnesium oxide is the form you get when you buy the cheapest bottle on the shelf — and the form behind most stories of magnesium "not working" or upsetting the stomach. It deserves a fair hearing: it is not useless, it is just widely used for the wrong job. This page explains both jobs. For magnesium overall (deficiency, daily needs, all forms compared), see the complete magnesium guide.
Key takeaways
- ~60% elemental magnesium — the most concentrated form, which is why pills can be small and cheap.
- But very poorly absorbed: about 4% in a human comparison,1 and no better than placebo in a 60-day trial.2
- What is not absorbed becomes a laxative — its genuinely evidence-backed use.3
- For correcting low magnesium, choose citrate or glycinate instead.
What it is
Magnesium oxide is the simplest magnesium compound there is: one magnesium, one oxygen — essentially a mineral, produced in bulk at very low cost. Its headline number is elemental content: about 60% of the salt’s weight is magnesium, four times more than citrate. That is why oxide tablets can be small while promising a large dose, and why the form dominates budget supplements and many multivitamins. The catch is that a label milligram is not a bloodstream milligram — which brings us to absorption.
Absorption — the weak point
Oxide dissolves poorly in the digestive tract, and the human data are blunt about the consequence. In a study of commercial magnesium products, fractional absorption of oxide was about 4%, significantly below the chloride, lactate and aspartate salts it was compared against.1 In a 60-day randomised, double-blind trial against citrate and an amino-acid chelate, oxide supplementation was indistinguishable from placebo on absorption measures.2
The other 96% does not simply vanish — it stays in the gut, drawing in water. That is why oxide is the most laxative of the common forms, and why a bargain bottle so often ends in cramps and an early bathroom visit rather than better magnesium status.
What it’s actually good for
Used deliberately, the laxative effect is the point. Magnesium oxide is a classic osmotic laxative, and it holds up in modern testing: in a randomised placebo-controlled trial in adults with chronic constipation, magnesium oxide improved bowel movements and symptoms — performing comparably to senna, a standard stimulant laxative.3 It is also an old-fashioned antacid for occasional heartburn. Those are honest, evidence-backed uses. What oxide is not good for is the job most people buy it for: correcting a low magnesium intake.
Dose
For constipation, follow the dose on the product (trials in adults commonly use around 1.5 g magnesium oxide/day, adjusted to effect) and take it with a full glass of water. Occasional use is the idea — constipation that persists beyond a couple of weeks belongs with a doctor, not a bigger dose. If your goal is magnesium repletion, do not chase it with oxide at all: at ~4% absorption almost everything you swallow ends up as laxative, and the well-absorbed forms cost only a little more.
Side effects & safety
Diarrhoea and stomach cramps are the expected effect, not a surprise. Because laxative doses of oxide are larger than ordinary supplement doses, the standard warning carries extra weight here: people with impaired kidney function can accumulate magnesium to dangerous levels and should not use magnesium laxatives without medical guidance. Keep 2–4 hours between magnesium and certain antibiotics, bisphosphonates and thyroid medication — full list in the main guide.
References
Adult research summary for education, not a prescription. These are food supplements, not treatments for disease.
- Firoz M, Graber M. Bioavailability of US commercial magnesium preparations. Magnes Res. 2001;14(4):257–262. PMID 11794633
- Walker AF, Marakis G, Christie S, Byng M. Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnes Res. 2003;16(3):183–191. PMID 14596323
- Morishita D, Tomita T, Mori S, et al. Senna versus magnesium oxide for the treatment of chronic constipation: a randomized, placebo-controlled trial. Am J Gastroenterol. 2021;116(1):152–161. PMID 32969946
This article summarises nutrition science for education. It is not a substitute for individual medical advice; consult a professional before starting a supplement, in pregnancy, in kidney disease, or alongside medication.