Abstract. The importance of magnesium and the positive effects of magnesium substitution in various diseases are extensively documented [ 1 , 2 ]. Magnesium compounds are widely used as medicinal and dietary supplements. The effectiveness of oral magnesium substitution for the treatment of magnesium deficiency is beyond controversy. However, the scientific seriousness of oral magnesium substitution has become questioned for several years through intensive marketing of magnesium-containing sprays for topical application. In both, specialist and lay press and on the internet, there are increasing numbers of articles claiming the effectiveness and superiority of magnesium oil over an oral magnesium intake. Magnesium oil is no real oil but a concentrated magnesium chloride solution which is sprayed on the skin. The transdermal absorption of magnesium in comparison to oral supplementation is presented as being more effective on the one hand due to nearly 100% absorption and on the other hand as it is associated with fewer side effects because of bypassing the intestinal tract. For proving this, various publications are mentioned.