Chemotherapy overdoses are very rare occurrences in which a patient has a severe reaction to a chemotherapy drug or when an excessive dose is accidentally administered. Symptoms of a chemotherapy overdose can be severe and may include cardiac problems, such as the heart stopping, or loss of consciousness. 

Overdoses during chemotherapy may be treated with medication to reverse the effects of the drugs. In 2015, the FDA approved Vistogard (uridine triacetate) to reverse the effects of the chemotherapy drugs 5-fluorouracil or capecitabine. Drug classes that may be administered by a physician include cytoprotectant agents.