Supplementation of various substances 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, whether it can be compensated by diet. Magnesium (Mg) deficiency has been associated with cardiovascular diseases, hypertension, stroke, certain neuropsychiatric and metabolic conditions. Hypomagnesemia is above-average in alcoholism; however, alcoholics should not be a priori assumed to have Mg deficiency. Mild depletion does not necessarily require specific therapy. Wherever possible, the oral route of supplementation is preferable. The parenteral route is mandatory in severe Mg deficiency. Hypermagnesemia may result from excessive supplementation. Intravenous infusions of Mg-containing solutions and some other invasive procedures have been used in the former Soviet Union without sufficient indications. The infusion therapy has been recommended also in moderately severe alcohol withdrawal syndrome. In conditions of suboptimal procedural quality assurance, endovascular and other invasive manipulations can lead to the transmission of viral hepatitis, which occurred to treated alcoholic patients. A combination of viral and alcoholic liver injury is unfavorable. It has been suggested to include Mg in routine blood ionograms. Mg contents in different foodstuffs should be taken into account in patients at risk of deficiency for better adjustment of diets.