Magnesium absorbed via skin bypasses the digestive system and does not cause diarrhea. When oral magnesium is taken in high concentration it attracts excessive water into the stool. You can do this for the laxative effect if needed, but note that the magnesium is expelled too quickly in this case for any significant amount to transit through the gut wall. The gut wall absorbs magnesium ions optimally when in lower concentrations, such as in foods and mineral water. If high amounts of magnesium are required, then the transdermal pathway is a better alternative, as the epidermal reservoir can hold the nutrients until fully absorbed internally.