Supplementation of various substances (metabolites, microelements, vitamins) is sometimes recommended without sufficient indications. To decide whether a supplementation is needed, the question should be answered whether there is a deficiency, and if there is, if it can be compensated by diet. Magnesium (Mg) deficiency has been associated with cardiovascular diseases, certain metabolic and neuropsychiatric disorders. Hypomagnesemia is above-average in alcoholism; however, alcoholics should not be a priori assumed to have Mg deficiency. Mild depletion does not necessarily require supplementation. The parenteral route is mandatory in severe Mg deficiency. Hypermagnesemia may result from excessive supplementation. Intravenous infusions of Mg-containing solutions have sometimes been used in alcoholics without sufficient indications. In conditions of suboptimal procedural quality assurance, endovascular and other invasive manipulations can lead to transmission of viral hepatitis. It has been suggested to include Mg in routine blood ionograms. The contents of Mg in different foodstuffs should be taken into account in patients at risk of Mg deficiency to better manage the diet.