Magnesium is generally safe with few to no side effects in most people; however, in some people it may produce diarrhea, stomach upset, vomiting and nausea, according to WebMD. Safe doses of magnesium for adults are under 350 milligrams per day.
When taken in large doses, magnesium may build up in the body, which is potentially unsafe as it can cause low blood pressure, irregular heartbeat, slowed breathing and confusion, states WebMD. In severe cases it can lead to coma and death. Pregnant or breast-feeding women should consult a doctor before taking magnesium. Individuals with heart block should not take high doses, and kidney patients should not take magnesium, as damaged kidneys cannot clear the mineral from the body properly.