Hypermagnesemia: Excess magnesium. Persons with impaired kidney function should be especially careful about their magnesium intake because they can accumulate magnesium, a dangerous (and sometimes fatal) situation. According to the National Academy of Sciences, the Recommended Dietary Allowances of magnesium are 420 milligrams per day for men and 320 milligrams per day for women. The upper limit of magnesium as supplements is 350 milligrams daily, in addition to the magnesium from food and water. Too much magnesium is hypermagnesemia.