The etiology of undernourishment in cancer patients is multifactorial: tumor-related mechanisms (such as obstruction, metabolic abnormalities, and functionality changes) in addition to the influence of anticancer therapies, which can induce or worsen undernutrition. The evident role of undernutrition in cancer treatment outcomes suggests the need of considering nutritional status when evaluating anticancer drugs. In order to merge the available data and offer researchers and clinicians a global view of this phenomenon, the present manuscript reviews on a drug-by-drug basis the undernutrition-related pharmacokinetic and pharmacodynamic aspects of anticancer treatments. This review notes interesting trends in the relationship between undernourishment and pharmacokinetics across studies, and indicates that dosing modifications of these drugs may be necessary to optimize chemotherapeutic treatments. Furthermore, this review has compiled evidence regarding undernourishment's capacity of enhancing treatment-related myelosuppression, cardiotoxicity, ototoxicity, neurotoxicity, and malignancies.