Composition comprises at least one omega -3 fatty acid of eicosapentaenoic (EPA) acid and/or docosahexaenoic acid (DHA), for improving the efficacy of chemotherapy or radiotherapy and/or in the prevention or reduction of the chemotherapy or radiotherapy-induced side effects in a patient suffering from cancer. The composition is administered to the patient before initiating a cycle of chemotherapy or radiotherapy. ACTIVITY : Cytostatic; Gastrointestinal-Gen; Antianemic; Hepatotropic; CNS-Gen; Cardiant; Antiinflammatory; Immunostimulant. MECHANISM OF ACTION : None given.