The coexistence of two or more diseases in one patient, especially in old age, is a common situation. The problem of drug effects, specifically speaking, their interaction and impact on the second condition still remains not fully explored and thus is very interesting. At the same time, it is commonly known that depression and cancer are some of the most frequently met disorders and following the WHO forecasts, their prevalence will constantly increase. In addition, it is worth noting that both the psychological and physical health of the patient suffering from cancer, are essential for proper co-operation, especially when it comes to adoption of recommended drugs. The case is aggravated by the fact that no standards of conduct in case of coexistence of the two diseases have been established and the results of research are very often contradictory. Therefore the issue discussed in this study seems to be very important and still valid. In this monograph the problem of the influence of antidepressant pharmacotherapy on the course of disease will be presented.