Magnesium oxide can inhibit the full effect of certain medications, such as tetracycline, when the medications are taken together. To prevent this, magnesium oxide should be taken two to three hours before or after taking other medications. This supplement also should not be taken with certain antacids because the interaction between the drugs can cause serious side effects.
Side effects of magnesium oxide include itching, possibly with a rash or hives, fatigue, both physical and mental, dizziness and nausea, and mood swings. Side effects range from mild to serious, though most people taking the supplement do not develop, or are not bothered by, side effects. If a person does develop side effects, he or she should consult a medical professional.
It is possible to overdose on magnesium oxide. Overdosing causes the heart to slow and blood pressure to lower. The person may feel drowsy or nauseous to the point of vomiting. Medical attention should be sought immediately if an overdose is suspected.