Background High values of endogenous levels of magnesium (Mg) in the body and mechanisms of homeostasis regulation make it difficult to assess the bioavailability of these drugs. The aim of this study was to assess the Mg concentration in blood in volunteers and in erythrocytes in patients with hypomagnesemia. Methods The study included 20 healthy volunteers and 62 patients with chronic heart failure (CHF) I–III functional class (FC) NYHA classification. We studied the composition of Mgorotate and Mgorotate plus potassium (К)orotate. Blood sampling was carried out at 8 a.m. and within 10 h after administering the drugs. Measurement of Mg pharmacokinetic parameters: AUC (concentration of the active substance-time), and Cmax (maximum concentration) in volunteers and measurement of the concentration of Mg in erythrocytes of patients. Results The results indicated that both the AUC in volunteers and concentration of Mg in erythrocytes of patients are comparable, and the differences are not statistically significant. Conclusions The study showed that the standard method of calculating the AUC (total serum Mg) is insufficient for comparative evaluation of Mg absorption due to the high levels of its endogenous content and a small increase in concentration after taking the drugs. It is advisable to assess the concentration of Mg in the red blood cells of patients.