We’ve analyzed results of the therapeutic drug monitoring (TDM). TDM is a routine investigation into the compulsory health insurance system and more than 3 000 TDM-tests are performed annually. The most demanded TDM was the monitoring of antiepileptic drugs, cyclosporin, tacrolimus; rarely were monitored digoxin and antibacterial agents. It turned out that more than % of patients with the application of standard doses of antiepileptic drugs do not reach the therapeutic concentrations of these drugs in the blood of patients. In some patients were observed excess of the therapeutic concentration range: for valproate in 15% of cases, for carbamazepine in 7% of cases. Patients treated with cyclosporine in 9%, and tacrolimus in 17% of cases required a dose adjustment. It is found that the application of various drugs containing the same drug substance in the blood concentration may vary considerably.