Calcium is needed for blood clotting, but magnesium keeps the blood flowing freely. Calcium Magnesium: a matter of balance Magnesium is necessary for muscles to relax.  Magnesium deficiency can be caused by a number of stresses on the body, including-but not limited to-lack of adequate dietary magnesium, emotional stress, some drugs (diuretics, antibiotics, oral contraceptives, insulin, cortisone), heavy exercise, diabetes, gastrointestinal disorders and too much calcium in the diet.