Background Despite multiple reports of chemotherapy overdoses (ODs) in human and veterinary medicine, anthracycline ODs have been described infrequently. Hypothesis/Objectives Describe toxicities, treatments, and overall outcome after anthracycline OD in dogs. Animals Twelve mitoxantrone (MTX) and 4 doxorubicin (DOX) ODs were evaluated. Methods Multicenter retrospective analysis. The American College of Veterinary Internal Medicine oncology and internal medicine listservs were solicited for cases in which a chemotherapy OD occurred. Results Sixteen anthracycline cases were collected. Anthracycline ODs occurred because of an error in chemotherapy preparation (n = 9), or dose miscalculation (n = 7). The overall median OD was 1.9× (range, 1.4‐10×) the prescribed amount. Most ODs were identified immediately after drug administration (n = 11), and the majority of patients were hospitalized on supportive care (n = 11) for an average of 8 days (range, 3‐34 days). Adverse events after the OD included neutropenia (94%), thrombocytopenia (88%), anemia (63%), diarrhea (63%), anorexia (56%), vomiting (38%), lethargy (31%), and nausea (25%). Two patients did not survive the OD. High grade neutropenia was common and did not appear to be mitigated by the administration of filgrastim. Conclusions and Clinical Importance All patients received supportive care after identifying the OD and death was uncommon. Further evaluation is needed to determine ideal therapeutic guidelines anthracycline OD.