Because anticancer drugs are cytotoxic for normal as well as neoplastic cells, the range of unwanted effects that accompanies their use is broad. Many of the side effects are potentially life-threatening or seriously debilitating. Many are similar to, and readily confused with, direct or indirect (paraneoplastic) consequences of the cancer itself. Recognition of drug side effects is vital for optimal patient care, because early withdrawal of the offending agent and institution of appropriate treatment have the potential to significantly reduce the overall morbidity and mortality associated with the diagnosis of cancer.