Definition of magnesium oxide. : a white highly infusible compound MgO used especially in refractories, cements, insulation, and fertilizers, in rubber manufacture, and in medicine as an antacid and mild laxative.Click to see full answer. Also question is, what is magnesium oxide used for?Magnesium is important for many systems in the body especially the muscles and nerves. Magnesium oxide is used as a supplement to maintain adequate magnesium in the body. Magnesium oxide is also used as an antacid to treat indigestion, or as a laxative to relieve occasional constipation.Likewise, which is better magnesium citrate or magnesium oxide? Orally, magnesium citrate is the best absorbed form (but it’s bonded to a big molecule so there is a smaller amount of magnesium by weight). Mg oxide is the most poorly absorbed form but has the highest Mg per weight, so actually you may get more elemental magnesium out of the same dose of Mg oxide vs. Regarding this, is it safe to take magnesium oxide daily? It’s recommended to only take a daily magnesium supplement that provides more than 350 mg while under medical supervision. Though magnesium toxicity is rare, taking certain magnesium supplements at high doses may cause diarrhea, nausea, and abdominal cramping.Is magnesium oxide bad?Upset stomach and diarrhea are the most common side effects of magnesium oxide. Taking magnesium oxide with food can reduce stomach troubles. If side effects are persistent or if they worsen, it is best to talk to your doctor. Most people using magnesium oxide supplements do not have serious side effects.