Hi. There's a recommended daily dosage of magnesium oxide, this is the most frequently used magnesium supplement. A general recommended dosage is about 400 to 500 milligrams a day. The common side effect of magnesium is abdominal cramping, vomiting, and diarrhea. The potential problem is that magnesium can build up in your body, so if you continue taking in a high dosage, it may lead to serious side effects like irregular heartbeat, unsafe low blood pressure, slowed breathing, coma. And, magnesium is associated with low blood pressure.