A compound that disrupts normal metabolism by mimicking a natural metabolite closely enough to enter biochemical pathways but differing enough to jam them. Many cancer and antimicrobial drugs work this way: methotrexate masquerades as folate to halt DNA synthesis, and fluorouracil impersonates a building block of RNA and DNA. In nutrition, the concept appears in antinutritional factors, natural food components that act as mild antimetabolites, such as the avidin in raw egg white that binds biotin so tightly the vitamin becomes unavailable. Understanding antimetabolites illuminates both therapy and toxicity: the same mimicry that makes a drug effective makes accidental exposure dangerous, and the dose determines whether a metabolic impostor heals or harms.