Venlafaxine (VEN) is considered to be one of the most effective antidepressants. It belongs to the group of serotonin (5-HT) and noradrenaline (NA) reuptake inhibitors (SNRIs). NA and 5-HT have receptors on the surface of platelets and are involved in platelet aggregation. In this case study, we present the case of a patient treated for one of the types of myeloproliferative neoplasm (MPN), essential thrombocythemia (ET), in whom VEN was added to pharmacotherapy during the treatment of a severe episode of depression with psychotic symptoms. We observed a gradual reduction in platelet count when increasing the dose of VEN. We also present a narrative review of literature about the effect of VEN on platelet counts and activity. We conclude that, in the group of patients taking VEN, attention should be paid to the rare adverse effect of a decrease in the number of platelets.