Major depression disorder (MDD) is a pervasive mood altering illnesses affecting energy, sleep, appetite, libido and the ability to function. Optimal treatment necessitates long-term administration of antidepressants. The pharmacokinetics (PKs) of these drugs are highly variable between patients which may result from drug interactions, food, renal or hepatic impairment, age, gender, smooking or genetic polymorphism of the drug metabolizing enzymes. These high interindiviual PK variabilities are therefore prone to the occurence of sub- or supratherapeutic plasma concentrations when standard doses are administered. The two selected antidepressant drugs in this thesis were venlafaxine (V) and mirtazapine (M). The application of therapeutic drug monitoring (TDM) for phenotyping and real time PCR for genotyping was found to be a proper way of performig a high degree of pharmaceutical care to depressed patients by adjusting the dose of administered drug and thus improving therapeutic efficacy and reducing risk of adverse effects of an antidepressive pharmacotherapy with V or M.