The intestine also responds to high magnesium levels in the blood by absorbing less magnesium from the gut as a secondary backup mechanism. However, elderly patients or patients with kidney disease can't handle ingested magnesium as well, resulting in hypermagnesemia (too much magnesium in the blood). Side effects of high magnesium levels include nausea, vomiting, lethargy, low blood pressure, low pulse rate, difficulty breathing, and in extreme cases, coma and death. Even in younger patients with normal kidneys, magnesium sulfate should not be used at a dose of more than 10 gm per day or for more than one week without a doctor's supervision.