This document explains the role of drugs in CAIE IGCSE Biology, focusing on their effects, metabolism, and the issue of antibiotic resistance. Drugs are defined as substances that alter or influence chemical reactions in the body, with metabolism occurring primarily in the liver. Antibiotics are described as medicines that kill or inhibit bacteria by disrupting essential processes such as cell wall formation, DNA replication, or protein synthesis, without harming human cells. However, antibiotics are ineffective against viruses, since viruses lack cell walls and rely on host cells for reproduction. The guide highlights the growing problem of antibiotic resistance, where bacteria develop mutations that provide resistance and pass these traits to offspring through binary fission. Resistant strains such as MRSA (Methicillin-resistant Staphylococcus aureus) are discussed as serious health concerns. The importance of responsible antibiotic use is emphasized—only taking them when necessary and completing the prescribed course to prevent resistance development. This resource helps students understand both the beneficial role of antibiotics and the dangers of resistance, linking microbiology to real-world health challenges.