A 4-year-old, entire, male boxer diagnosed with a T-cell multicentric lymphoma was referred for managing cyclophosphamide overdose complications after a prescription mistake during a 19-week WM-CHOP rescue protocol (vincristine, cyclophosphamide, doxorubicine, prednisolone). Adverse events included myelosuppression with grade IV neutropenia and thrombocytopenia, fever, haematuria, rectorrhagia and petechiae. With supportive care and the use of granulocyte-macrophage colony-stimulating factor, the patient recovered completely 23 days after the overdose. Once recovered, the protocol continued with doxorubicin as a single agent. The dog was euthanased due to lymphoma relapse without overdose sequelae. This case report describes for the first time, a recovery after cyclophosphamide intoxication produced immediately after the administration of vincristine in a cancer patient. Therefore, vincristine at therapeutic dose did not seem to add toxicity synergistically in this patient. Security strategies have to be put in place in all the centres to minimise the risk from these life-threatening accidents.