Magnesium sulfate, commonly known as Epsom salt, is a mineral. It works by replacing magnesium in the body and increasing water in the intestines.Magnesium sulfate can be used orally as a laxative to relieve occasional constipation, and to treat low levels of magnesium.Known as hypomagnesemia, low magnesium levels can be caused by alcoholism, severe diarrhea, malnutrition, or high calcium levels (hypercalcemia).on't use magnesium sulfate if it contains particles or is cloudy or discolored. Magnesium Sulfate as a Soak: You can also use magnesium sulfate as an Epsom salt soak. Dissolve into a large amount of water in a large bowl, bucket, foot tub, or bath tub.