BDNF and major depressive disorder: In conjunction with the above, the variability between studies in relation to the age of the patients, the type of diagnosis, the different diagnostic tools used, the age of onset of MDD, the number of depressive episodes, the dose and duration of therapy, different samples with a history of previous treatment vs. virgin patients to treatment and the different methods of measuring BDNF are some of the limitations that we face in the analysis of our results.