The most common side effects of magnesium oxide are stomach upset and diarrhea, states WebMD. Cramping is also a side effect of magnesium oxide, according to MedlinePlus.
Severe side effects of magnesium oxide include unusual fatigue, nausea and weakness, explains MedlinePlus. Black and tarry stools, slow reflexes and vomit that resembles coffee grounds are also severe side effects of magnesium oxide, states Drugs.com. Some patients experience severe allergic reactions to magnesium oxide and have side effects that include rashes, itching and hives, states WebMD. Other side effects that indicate an allergic reaction include tightness in the chest and swelling of the face, tongue, lips or throat.