Adverse drug reactions (ADRs) range from predictable pharmacologic effects to rare immune-mediated or idiosyncratic responses that can cause significant morbidity. Understanding the mechanisms behind ADRs—pharmacokinetic interactions, hypersensitivity pathways, and genetic susceptibilities—is key to prevention and safe prescribing. This book explains the biological and clinical bases of ADRs, grouping reactions by mechanism and organ system involvement. Practical sections cover risk assessment, monitoring strategies, deprescribing, and stepwise management of acute drug reactions. It includes guidance on reporting systems, pharmacovigilance, and how to counsel patients about risks and alternatives. Designed for clinicians, pharmacists and trainees, the text balances mechanistic depth with actionable clinical algorithms.