Methodological problems of the influence of antiepileptic drugs (AED) on epileptic seizures are characterized. An analysis of 505 patients with diagnosis of pharmacoresistant epilepsy has been conducted. In 187 (37%) patients the causes of treatment resistance were as follows: overtreatment (73 patients; 14,5%), inadequate AED prescription potentially predicted effects - (69 patients; 13,7%), paradoxical reactions to AED (45 patients; 8,9%). Inadequately prescribed were mostly carbamazepine and phenytoin and the drug with paradoxical effect - valproate.