Compositions having at least one flavonoid of natural or synthetic origin in association with melatonin to increase the effectiveness of chemotherapeutic treatments used in human and veterinary medicine for the treatment of tumours are described. The compositions are in particular for the treatment of tumours resistant to the chemotherapeutic agents currently in use. The at least one flavonoid is selected from rutin, oxerutin, diosmin and hesperidin, preferably rutin.
