According to Daniel Reed, author of Tao of Detox, magnesium sulfate is rapidly excreted through the kidneys and therefore is difficult to assimilate. Dr. Mark Sircus suggests in Chapter 14 "Magnesium Chloride vs. Magnesium Sulfate" of Transdermal Magnesium Therapy (2007) that the rapid rate of excretion in part explains why the effects of Epsom Salts do not last long, and why you need more magnesium sulfate than magnesium chloride to get similar results. Magnesium chloride is more easily assimilated and metabolized in the human body (Transdermal 210).
Dr. Jean Durlach of the University of Paris writes concerning the relative toxicities between magnesium chloride and magnesium sulfate, "The reason of the toxicity of pharmacological doses of magnesium using the sulfate anion rather than the chloride anion may perhaps arise from the respective chemical structures of both the two magnesium salts. Chemically, both magnesium sulfate (MGSO4) and Magnesium chloride (MgCl2) are hexa-aqueous complexes. However, MgCl2 crystals consist of dianions with magnesium coordinated to the six water molecules as a complex, [Mg(H2o)6]2+ and two independent chloride anions, Cl-. In MgSO4, a seventh water molecule is associated with the sulfate anion, [Mg(H2O)6]2 +[SO4. H2O]. Consequently, the more hydrated MgSO4 molecule may have chemical interactions with paracellular components, rather than with cellular components, presumably potentiating toxic manifestations while reducing therapeutic effect."
Dr. Durlach concludes: "MgSO4 is not always the appropriate salt in clinical therapeutics. MgCl2 seems the better anion-cation association to be used in many clinical and pharmacological indications."
Dr. Mark Sircus notes, for example, that a search of studies in Medline for magnesium sulfate used in pregnancy indicates that high doses can be toxic, and even lethal, for newborns. However, he notes that lower doses appear safe. He also notes that sulfur is an important mineral for health. Individuals who require sulfur may therefore benefit from magnesium sulfate. For example, autistic children can benefit from Epsom Salts because they are deficient in sulfate. Nevertheless, magnesium chloride is a superior form due to greater uptake and lower toxicity.
Dr. Sircus asserts that magnesium sulfate can save a life during an emergency as rapidly as magnesium chloride, but that magnesium chloride truly is a universal medicine, and magnesium sulfate is not. He concludes that magnesium sulfate is a "close cousin [of magnesium chloride] whose effect, form, and toxicity demands it be used in special applications when sulfur is needed" (Transdermal 211-12).
The practical effect of magnesium chloride solution (magnesium oil) is "similar to Epsom Salts , but much stronger" and takes the experience of using transdermal magnesium to a new level (Transdermal 252-53).
"Until now it was thought that the best forms of supplemental magnesium were the ones chelated to an amino acid (magnesium glycinate, magnesium taurate) or a krebs cycle intermediate (magnesium malate, magnesium citrate, magnesium fumarate). These forms seem to be better utilized, absorbed, and assimilated. Some have correctly advised to stay away from oral intake of inorganic forms of magnesium like magnesium chloride (taken orally) or magnesium carbonate because they may not be absorbed as well and may cause gastric disturbances. But now we have a magnesium chloride lotion/bath salt that can be applied directly to the skin so dosage levels can be brought up safely to high levels without diarrhea and problems with absorption" (Transdermal 253-54).
The form of magnesium chloride preferred by Dr. Sircus is magnesium oil, specifically Ancient Minerals magnesium oil and Ocean Minerals magnesium oil. These are available from many sources, including Integrated Health:
Epsom salts work. However, magnesium chloride offers a wider range of benefits, greater effectiveness, and better safety. Magnesium oil may cost a bit more, but consider that you will need less of it, and that it is a universal, natural medicine that can provide healthy benefits to almost everyone.