Magnesium Oxide

Magnesium oxide is the cheapest and most common magnesium form, with a very high elemental magnesium content but poor absorption (only a small fraction is taken up). Much of it stays in the gut, which is why it works well as a laxative or antacid but less well for raising magnesium levels.

Because it is poorly absorbed and draws water into the bowel, magnesium oxide is most useful for occasional constipation and as an antacid for heartburn, rather than for correcting a magnesium deficiency, where better-absorbed forms are preferred.

A 400 mg magnesium oxide tablet provides a lot of elemental magnesium on paper, but little is absorbed. For constipation it is taken as directed with water; for supplementation, a more absorbable form like glycinate or citrate is usually a better choice.

Yes, that is its main effect at higher doses, since the unabsorbed magnesium pulls water into the intestines. If you want magnesium without a laxative effect, choose magnesium glycinate or another chelated form.

Magnesium oxide is the most common (and cheapest) magnesium form found in OTC supplements and multivitamins — but among the LOWEST bioavailability. The high elemental magnesium content (60% by weight) is misleading because absorption is poor (~4%) and the unabsorbed portion causes osmotic diarrhea.