Lacosamide is a modern antiepileptic drug used as monotherapy or adjunctive therapy in the treatment of partial seizures with or without secondary generalization in adults, adolescents, or children from 4 years of age. According to ILAE, the concentrations that achieve optimal disease control for most patients are in the range of 10.0-40.0 μmol / L. The aim of this study was to measure and evaluate lacosamide concentrations in serum samples of patients on antiepileptic therapy. Steady-state lacosamide concentration was determined on an HPLC Prominence Shimadzu device using an internal standard. Measured concentrations were analyzed by sex and age and it was found that gender and age did not affect lacosamide concentrations, ie that lacosamide concentrations did not differ statistically significantly between groups of the opposite sex and groups divided by age: pediatric population (younger <7 years), 7-18, 19-45, 46-65 years and geriatric population (older> 65 years). Patients were also classified into groups according to the number of additional antiepileptics in therapy and the presence of inducers and enzyme inhibitors in polytherapy and the analysis showed that lacosamide concentrations differ significantly in patients on polytherapy with an inducer / inhibitor of metabolism. Concentrations of lacosamide in polytherapy with an inducer of metabolic enzymes are significantly lower compared to values without inducer / inhibitor. 10% of the examined patients, in addition to the antiepileptic drug, also take an antidepressant / antipsychotic / anxiolytic for which no effect on lacosamide concentrations has been proven. Analysis of lacosamide concentrations relative to the recommended reference interval showed that 70% of patients achieved the desired concentrations. Our results showed that gender and age do not affect the concentration of lacosamide, but the type of other antiepileptic therapy (inducers / inhibitors) has a significant effect because it leads to changes in the pharmacokinetics of lacosamide, so there is a need for individualization of therapy and therapeutic monitoring of lacosamide. The dose of lacosamide is determined for each patient according to age, BMI and renal function, but determining the concentration of the drug that allows optimal control of therapy and minimal side effects can significantly contribute to treatment especially in polytherapy with drugs that affect metabolism.