Not a botanical (inorganic magnesium salt, MgO) · mineral
Evidence tier: moderate
Magnesium oxide is magnesium bound to oxygen, a form that hardly dissolves in water. In one study of volunteers, about 4 percent of it was absorbed.
Magnesium oxide is about 60 percent magnesium but dissolves and absorbs poorly, so most stays in the bowel and draws water in (an osmotic laxative effect).
Moderate for one use only, constipation. A 2023 meta analysis of 2 RCTs found 68 percent responded on oxide vs 19 percent on control, with about 3.7 more bowel movements a week (PMID 37243443); a 90 patient RCT found 1.5 g a day matched senna and beat placebo (PMID 32969946); the British Dietetic Association 2025 guideline makes a strong recommendation at 0.5 to 1.5 g a day for at least 4 weeks (PMID 41081513). Those doses exceed every supplemental upper limit. As a way to raise magnesium status it is poor: NIH ODS lists it among the less bioavailable forms, fractional absorption was about 4 percent in one study (PMID 11794633) and it was no better than placebo on blood levels over 60 days (PMID 14596323). ODS lists it among the forms that most often cause diarrhoea.
Constipation: 0.5 g a day to start, up to 1.5 g a day, for at least 4 weeks, with a pharmacist or doctor (BDA 2025). Antacid or supplement: 1 to 2 tablets of 400 mg a day, no more than 2 in 24 hours without a doctor (US OTC label).
Oxide is about 60 percent magnesium but poorly absorbed (about 4 percent fractional absorption in one study), so it is a poor way to raise magnesium status and the best studied form for constipation. Among the forms most reported to cause diarrhoea. Not with kidney disease unless a doctor advises. Space from tetracycline and quinolone antibiotics, bisphosphonates and levothyroxine as for all magnesium. [NIH ODS; PMID 11794633; DailyMed]
Onset: ~28 days
Pregnancy: caution
Breastfeeding: caution
Active compounds: elemental magnesium
Remedies using Magnesium Oxide · Browse by body system · Check it against what you take
*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.